fix: Missing line break on canonical request.

Fixes #2250.

- This is not a 100% reproducible bug, but it happens enough without this change, and hasn't been observed with this change.
- The Java implementation [includes this extra line break](https://github.com/googleapis/google-auth-library-java/blob/5e5fe411af88f00fd7fe09b1f4ca4d5d9417ecab/oauth2_http/java/com/google/auth/oauth2/AwsRequestSigner.java#L177).
- From looking at the [signature spec](https://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html) (see point 4), is still not entirely clear that this extra line break at the end of the canonical headers is actually specified in the requirements.

Author: amanda-tarafa

Src/Support/Google.Apis.Auth/OAuth2/AwsExternalAccountCredential.AwsSignedSubjectToken.cs

@@ -145,6 +145,7 @@ string GetHashedCanonicalRequest()
                         canonicalRequest.Append(headerPair.Value);
                         canonicalRequest.Append(newLine);
                     }
+                    canonicalRequest.Append(newLine);
 
                     // Now the list of headers that will be signed.
                     // See point 5 in https://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html