Permit getting list of composable templates

[DaveCTurner]

We document that GET /_index_template/... accepts a comma-separated
list of template names but in fact today this API accepts only a single
name or pattern. We expect folks sometimes to have enough templates to
want to be able to request a list of them, so this commit generalises
the API to accept a list as per the docs.

It also adjusts GET /_cat/templates to use this new feature to filter
the list of composable templates on the master rather than on the
coordinating node.

[elasticmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[dakrone]

@DaveCTurner we actually discussed this in the past and decided against adding functionality like this. The reason, if I recall, relates to the way we should handle partial failure of the request with and without wildcards. If the docs say we support that we should fix those so that they are correct.

For example, here we explicitly removed docs for comma-separated list of names for deletion: #70649

[DaveCTurner]

WDYT about keeping this functionality in the transport action and just blocking it in the REST action? GET _cat/templates doesn't have the same concerns about partial failures, but it is pretty inefficient to get all the composable templates from the master and filter them on the coordinating node.

[dakrone]

@DaveCTurner that works for me I think

[DaveCTurner]

Actually on reflection the change to GET _cat/templates to support multiple patterns as per the docs is also new in 7.16 (see #78829). In this branch I've reverted both APIs back to the old single-pattern behaviour and fixed the docs to match. I will use this PR to check I didn't miss anything in CI and then close this and open a fresh PR with a corrected description.

[DaveCTurner]

Closing this in favour of #78989