Update IndexTemplateMetaData to allow unknown fields

[martijnvg]

Update IndexTemplateMetaData to use ObjectParser
The IndexTemplateMetaData class used old parsing logic and was not
resiliant to new fields. This commit updates it to use the
ConstructingObjectParser and allow unknown fields.

Also a test was added that randomly generates a response that an ES node would returns and
this test verifies that HLRC can parse it. This gives us the test coverage and confidence that
new parsing code is working well.

Relates #36938

[elasticmachine]

Pinging @elastic/es-core-features

[hub-cap]

LGTM but i wrote 1/2 the code so id like an independent set of 👓

[dakrone]

LGTM

[jakelandis]

I don't think this can go back to 6.7 , since if I read this correctly... it also removes support for the deprecated "template" field, which Logstash (and possibly others) still uses on 6.x. Also we should probably add this as a breaking change and add to deprecation info API.

EDIT: this is client only code, so we don't need to add to deprecation info API

[jakelandis]

LGTM for 7.x only

However, I would encourage a test that explicitly tests that "template" is no longer supported.

I just realized this is only for the client code (not server side), so no need to add to breaking changes for server. I am OK with the client being more aggressive here then the server w/r/t to support for the deprecated field. (A test here would help to communicate that too).

[jakelandis]

@martijnvg - if you merge this is 7.x, could you please log a follow up issue to remove support for "template" in the server side parsers, targeted at 8.0 ?

[martijnvg]

@jakelandis The IndexTemplateMetaData.Builder#toInnerXContent(...) method never serializes the index patterns under the pattern key, always under the index_patterns key. Since what this method generates, is what the hrlc will parse, I think this PR can be backported to 6.7? I'm not saying we should, but there is no harm in backporting it?

The logic that with the pattern key is only used for reading xcontent from the client on the server side. This parsing code was copied to hlrc, but can safely be removed.

if you merge this is 7.x, could you please log a follow up issue to remove support for "template" in the server side parsers, targeted at 8.0 ?

Yes, I will open an issue for this.

[jakelandis]

@martijnvg - I see now, I missed this part: "IndexTemplateMetaData.Builder#toInnerXContent(...) method never serializes..." Thanks for explaining.

LGTM for 6.x too.