feat: Add support for context and multiple identity sources

[00Greenwood]

Description

This PR adds support for request authorizers to handle context based identity sources. But seeing as context is set by us, I am not throwing an error if context required is not set.
Also added support for request authorizers to handle multiple identity sources, all of them are now check and required for the authorization function to be called.

Motivation and Context

Following on from #1610, my personal project requires a request authorizer which use a context identity source along with a header identity source. AWS supports this by defining the identity sources as a list of comma separated strings XXX, YYY, ZZZ.

How Has This Been Tested?

Ran locally with my Serverless Project which has a request authorizer which requires context and header identity sources.
Added some integration tests for checking the new mixed case.

Screenshots (if appropriate):

[airdrummingfool]

@00Greenwood thanks for this PR! Support for multiple fields in identitySource is a big improvement to Serverless Offline.

I wanted to add a comment for anyone trying to use this with the authorizer payload version 2.0: the identitySource field in v1 is a String, but in v2 it is an Array. I had to add a small check and join to make my array identitySource work.

diff --git a/src/events/http/createAuthScheme.js b/src/events/http/createAuthScheme.js
--- a/src/events/http/createAuthScheme.js
+++ b/src/events/http/createAuthScheme.js
@@ -284,8 +284,9 @@ export default function createAuthScheme(authorizerOptions, provider, lambda) {
       'g',
     )
 
+    const identitySourceString = Array.isArray(authorizerOptions.identitySource) ? authorizerOptions.identitySource.join(',') : authorizerOptions.identitySource;
     const identitySourceMatches =
-      authorizerOptions.identitySource.matchAll(identitySourceRegExp)
+      identitySourceString.matchAll(identitySourceRegExp)
 
     const typesAndFields = []
     for (const match of identitySourceMatches) {