fix: Make APM optional in gatsby package

[AbhiPrasad]

This PR has two changes after I did some testing with gatsby (https://github.com/getsentry/develop)

1. fix: Make APM optional in gatsby package

If a user has @sentry/apm or @sentry/tracing, we will use it, otherwise just fallback to normal.

2. ref: Use idleTimout if no activities occur in idle transaction

If idleTransaction does not get any activities pushed in the idleTimeout specified we finish the transaction (it most likely points to a simple page with no requests going to happen).

We do this by setting a timeout on idle transaction creation. The moment we push an activity, we cancel this timeout and instead rely on the activity system to end the transaction. This is behaviour that was in @sentry/apm where it would fake push and pop an activity when creating an idle transaction.

[github-actions]

size-limit report

Path	Size	Loading time (3g)	Running time (snapdragon)	Total time
@sentry/browser - CDN Bundle (gzipped)	17.64 KB (+0.01% 🔺)	353 ms (+0.01% 🔺)	528 ms (-8.26% 🔽)	881 ms
@sentry/browser - Webpack	18.39 KB (0%)	368 ms (0%)	746 ms (+3.83% 🔺)	1.2 s
@sentry/react - Webpack	18.39 KB (0%)	368 ms (0%)	761 ms (-5.63% 🔽)	1.2 s
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	22.57 KB (+0.15% 🔺)	452 ms (+0.15% 🔺)	657 ms (-13.57% 🔽)	1.2 s

[getsentry-bot]

	Messages
📖	@sentry/browser bundle gzip'ed minified size: (ES5: 17.2373 kB) (ES6: 16.3535 kB)
📖	✅ TSLint passed

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