Support Envoy retry policies

[manigandham]

Automatic retries are a core feature of Envoy but the latest v0.50x doesn't support envoy_override anymore and there are no appropriate settings in the Mapping config.

This should be part of the Mapping API. It can support a basic subset until envoy_override is supported again or match the Envoy API.

[caiobegotti]

As a side comment, what is the expectation about supporting something like envoy_override again after 0.50.x? I can't upgrade it myself due to that, that's how I landed on this ticket here :-(

[richarddli]

The v2 YAML configuration that Envoy uses is quite a bit more complex than the v1 configuration we used to support.

So, it's on the list, but it will be quite different. In the meantime, though, the Ambassador code is much cleaner, so would welcome direct contributions to the code base to support more features! :)

[richarddli]

Merged to master, will be released shortly