Update gSSP type to support props as a promise

[kara]

In a previous PR, getServerSideProps was altered to support returning
props as a promise. This change updates the TS types to permit promises
as well, so you can write type GetServerSideProps<Props> instead of
GetServerSideProps<Promise<Props>>. e.g.:

type Props = {
  data: string
}

export const getServerSideProps: GetServerSideProps<Props> = async (
  context
) => {
  return {
    props: (async function () {
      return { data: 'some data' }
    })(),
  }
}

Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not.
• Errors have helpful link attached, see contributing.md

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	kara/next.js types	Change
buildDuration	14.7s	14.3s	-337ms
buildDurationCached	3.6s	3.7s	⚠️ +58ms
nodeModulesSize	47 MB	47 MB	⚠️ +14 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	kara/next.js types	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.847	2.824	-0.02
/ avg req/sec	878.2	885.4	+7.2
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.505	1.499	-0.01
/error-in-render avg req/sec	1661.01	1667.41	+6.4

Client Bundles (main, webpack, commons)

	vercel/next.js canary	kara/next.js types	Change
745.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	26.6 kB	26.6 kB	✓
webpack-HASH.js gzip	1.45 kB	1.45 kB	✓
Overall change	70.4 kB	70.4 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	kara/next.js types	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	kara/next.js types	Change
_app-HASH.js gzip	979 B	979 B	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	329 B	329 B	✓
dynamic-HASH.js gzip	2.67 kB	2.67 kB	✓
head-HASH.js gzip	351 B	351 B	✓
hooks-HASH.js gzip	918 B	918 B	✓
image-HASH.js gzip	4.14 kB	4.14 kB	✓
index-HASH.js gzip	261 B	261 B	✓
link-HASH.js gzip	1.66 kB	1.66 kB	✓
routerDirect..HASH.js gzip	318 B	318 B	✓
script-HASH.js gzip	387 B	387 B	✓
withRouter-HASH.js gzip	320 B	320 B	✓
bb14e60e810b..30f.css gzip	125 B	125 B	✓
Overall change	13 kB	13 kB	✓

Client Build Manifests

	vercel/next.js canary	kara/next.js types	Change
_buildManifest.js gzip	492 B	492 B	✓
Overall change	492 B	492 B	✓

Rendered Page Sizes

	vercel/next.js canary	kara/next.js types	Change
index.html gzip	539 B	539 B	✓
link.html gzip	552 B	552 B	✓
withRouter.html gzip	533 B	533 B	✓
Overall change	1.62 kB	1.62 kB	✓

---

Webpack 4 Mode (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	kara/next.js types	Change
buildDuration	11.8s	12.1s	⚠️ +211ms
buildDurationCached	5s	4.8s	-210ms
nodeModulesSize	47 MB	47 MB	⚠️ +14 B

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	kara/next.js types	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.915	2.891	-0.02
/ avg req/sec	857.71	864.89	+7.18
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.526	1.548	⚠️ +0.02
/error-in-render avg req/sec	1637.78	1615.25	⚠️ -22.53

Client Bundles (main, webpack, commons)

	vercel/next.js canary	kara/next.js types	Change
16.HASH.js gzip	186 B	186 B	✓
677f882d2ed8..HASH.js gzip	14.1 kB	14.1 kB	✓
framework.HASH.js gzip	41.9 kB	41.9 kB	✓
main-HASH.js gzip	13.8 kB	13.8 kB	✓
webpack-HASH.js gzip	1.19 kB	1.19 kB	✓
Overall change	71.2 kB	71.2 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	kara/next.js types	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	kara/next.js types	Change
_app-HASH.js gzip	964 B	964 B	✓
_error-HASH.js gzip	3.8 kB	3.8 kB	✓
amp-HASH.js gzip	552 B	552 B	✓
css-HASH.js gzip	333 B	333 B	✓
dynamic-HASH.js gzip	2.87 kB	2.87 kB	✓
head-HASH.js gzip	3.06 kB	3.06 kB	✓
hooks-HASH.js gzip	924 B	924 B	✓
index-HASH.js gzip	231 B	231 B	✓
link-HASH.js gzip	1.64 kB	1.64 kB	✓
routerDirect..HASH.js gzip	298 B	298 B	✓
script-HASH.js gzip	3.03 kB	3.03 kB	✓
withRouter-HASH.js gzip	295 B	295 B	✓
30809af5c834..565.css gzip	125 B	125 B	✓
Overall change	18.1 kB	18.1 kB	✓

Client Build Manifests

	vercel/next.js canary	kara/next.js types	Change
_buildManifest.js gzip	500 B	500 B	✓
Overall change	500 B	500 B	✓

Rendered Page Sizes

	vercel/next.js canary	kara/next.js types	Change
index.html gzip	585 B	585 B	✓
link.html gzip	597 B	597 B	✓
withRouter.html gzip	578 B	578 B	✓
Overall change	1.76 kB	1.76 kB	✓

Commit: 62e68a2