[v11] Deprecate OAuthProvider API get credential API that uses string provider ID

[ncooke3]

• Requires API review

Process:

1. Decorate existing ObjC API with @available(swift, introduced: 100.0) to make it ObjC-only and replace String types in ObjC-only API with NSString. This has the following result:

3. Copy the ObjC-only API and remove the @available(swift, introduced: 100.0) attribute, the @objc(...) decorator, and swap the NSString types for String. This should leave the current Swift API. Mark it deprecated (see PR diff for example deprecation comment).
4. Add new signature, swapping the providerID: String for providerID: AuthProviderID
5. I looked over the newly added Swift API and there was an opportunity to consolidate two API into one by using a param with default value. See commit diff in ed6370f

No changes are needed to ObjC API tests:

firebase-ios-sdk/FirebaseAuth/Tests/Unit/ObjCAPITests.m

Lines 420 to 428 in 63070a4

	FIROAuthCredential *c = [FIROAuthProvider credentialWithProviderID:@"id" accessToken:@"token"];
	c = [FIROAuthProvider credentialWithProviderID:@"id"
	IDToken:@"idToken"
	accessToken:@"accessToken"];
	c = [FIROAuthProvider credentialWithProviderID:@"id" IDToken:@"idtoken" rawNonce:@"nonce"];
	c = [FIROAuthProvider credentialWithProviderID:@"id"
	IDToken:@"token"
	rawNonce:@"nonce"
	accessToken:@"accessToken"];

Decisions:

1. Add docs to ObjC-only API and new Swift API, and don't add docs for the deprecated Swift API

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[ncooke3]

The "get credential" API are done. I was going to break up the migration into smaller PRs since the providerID: String is used in many places right now.