Release GAX 2.8.0
Release GAX 2.8.0

Changes:

- New methods for the x-goog-request-params header (need generator support)
- New ClientBuilderBase for REST-based client wrappers
- Add a UserAgent property in the gRPC ClientBuilderBase
- Improve gRPC ChannelOptions handling in builders and pools
- Expose equality helpers as GaxEqualityHelpers

googleapis/gax-dotnet,googleapis/gax-dotnet,jskeet/gax-dotnet,jskeet/gax-dotnet