feat(Core): add retryFunction in Rest/Grpc traits

[vishwarajanand]

Earlier we added support for retryFunction in GrpcRequestWrapper per #5770. With this change, we intend to surface those changes in GrpcTrait and RestTrait so that it can be consumed by respective implementations of ConnectionInterface directly.

Change

1. Exposing retry function into rest/grpc traits
2. Added tests for unit testing

Tests

google-cloud-php/Core % XDEBUG_MODE=debug vendor/bin/phpunit -c phpunit.xml.dist --filter="testSendsRequestWithRetryFunction"
PHPUnit 8.5.31 by Sebastian Bergmann and contributors.

..                                                                  2 / 2 (100%)

Time: 2.87 seconds, Memory: 14.00 MB

OK (2 tests, 4 assertions)
google-cloud-php/Core %

ref: b/263775077
Note: After these changes are picked up by a release PR, we can make final change in Spanner to use these changes.