Add retries, deprecate PHP 7.3

[splittingred]

Just putting this up here as a draft for reference. I'm on other priorities at present; but this illustrates how to implement retries natively into grphp (without the need for an interceptor).

This has to be done both in the cURL-based strategies (H2Proxy and Envoy strategies) and in the gRPC native strategy; for the gRPC strategy we follow the specification here: https://github.com/grpc/proposal/blob/master/A6-client-retries.md

For the HTTP bridge filter ones (via Envoy or nghttpx), we should follow the gRPC native structure of the retry policies definition, but execute that in cURL. This is currently not finished in this PR.