[FEATURE] Add specifications for the learning-to-rank/ltr plugin

[Xtansia]

Is your feature request related to a problem?

The specification does not contain definitions for the Learning to Rank Plugin.

What solution would you like?

Specifications to be added for the plugin.

What alternatives have you considered?

As it's not a default installed plugin we may not want to add it to this repo, but it is owned under the OpenSearch Github Org.

[JohannesDaniel]

You can assign this ticket to me as it is a precondition for opensearch-project/opensearch-learning-to-rank-base#141

[JohannesDaniel]

As this is quite comprehensive, I need someone to agree on how to do it - whether to do it in many steps or all at once.

[nhtruong]

@JohannesDaniel We usually break something of this scale into multiple PRs. You can start with a small one with only 1 endpoint to get the feel of this first.

[JohannesDaniel]

@nhtruong created first PR, passed all checks:
#829

[nhtruong]

Wonderful. Thank you. Approved and merged!