DRIVERS-3207: Custom AWS credential providers execute earlier

[durran]

Updates the AWS auth spec to require drivers that implement custom AWS credential providers to use the custom provider before any other method.

Updates test wording to include the extra scenario for testing a custom provider when explicit credentials are also provided.

Node implementation: mongodb/node-mongodb-native#4656

Please complete the following before merging:

• Update changelog.
• Test changes in at least one language driver.
• Test these changes against all server versions and topologies (including standalone, replica set, and sharded
  clusters).

[durran]

#1838 (comment)

I have added a prose test for FLE in case 26. Case 15 I don't believe is relevant as it pertains to automatic credential fetching, which means nothing is provided. Even with the changes in this PR the order on automatic credential fetching is the same.

[durran]

#1838 (comment)

Added link.

[katcharov]

Would this be similar to

specifications/source/auth/auth.md

Lines 1298 to 1303 in 3dc84d1

	- OIDC_CALLBACK
	An [OIDC Callback](#oidc-callback) that returns OIDC credentials. Drivers MAY allow the user to specify an
	[OIDC Callback](#oidc-callback) using a `MongoClient` configuration instead of a mechanism property, depending on
	what is idiomatic for the driver. Drivers MUST NOT support both the `OIDC_CALLBACK` mechanism property and a
	`MongoClient` configuration.

? If so, consider using similar wording.