Shard Failure alternatives support

[l-trotta]

Originally reported in elastic/elasticsearch-java#408.
ShardFailure has 3 variants in the server code:

1. ShardSearchFailure, the one we currently map, commonly used by most Search responses
2. SearchFailure, which is a bit different (shard is nullable and there's status), and it's used by Scrollable responses
3. Failure, which uses underscores in most fields and adds primary, and is used by every response that extends ReplicationResponse (mainly Update, Delete and Index, but also some Bulk)

To support all 3 without creating specific classes and introduce breaking changes we have decided to have ShardFailure support all variants by using aliases and making additional fields nullable (with reason being the only non optional field that every variant supports)

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.

[l-trotta]

@Anaethelion, @flobernd, @miguelgrinberg checking if this breaks any generator before merging

[miguelgrinberg]

@l-trotta the Python DSL generator ignores the aliases, so this change causes no problems for me.

[pquentin]

Thanks! The change itself LGTM. We still need to wait confirmation for .NET and Go.

[Anaethelion]

@l-trotta This will only translates into alternatives field names within deserialization in Go, so you're good. 👍

[flobernd]

Since I suggested the use of aliases, LGTM from my side as well 😅

I'm handling it the same way as Go.