Add project_routing query param to _search and _async_search

[quux00]

Adds the project_routing query param to the _search and _async_search endpoints
but only in serverless. This param is not available in stateful ("stack").

[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]

@quux00 thanks! waiting for the server side PR to be merged. just to be sure, is the only value allowed _alias:something? would it make sense to have the user only input the customizable part of the string?

[quux00]

I see these on another PR:

@availability serverless stability=stable visibility=public

Do I need to add those? What are the other possible values?

[l-trotta]

they are optional, but nice to have. possible values:

• stability: stable, experimental, unknown
• visibility: public, private, feature_flag

in case of visibility=feature_flag you can also add an optional feature_flag={name of the feature flag}.
lastly there's since, which indicates (for stack) the version where the endpoint/field was introduced.
complete example:

elasticsearch-specification/specification/_global/search/SearchRequest.ts

Line 389 in 3a98596

* @availability stack since=8.4.0 visibility=feature_flag feature_flag=es.index_mode_feature_flag_registered