MB-35347: Add REST endpoint to retrieve synonym sources from index mapping

[CascadingRadium]

• This endpoint allows users to retrieve the names of synonym sources defined in an
  index mapping, enhancing accessibility and configurability for synonym-related features.

[abhinavdangeti]

@CascadingRadium Use v0.5.4 for this commit.

[CascadingRadium]

this must have been merged after the bleve merge 😅 since this requires uses

	synonymSources := make([]string, 0, indexMapping.SynonymCount())
	for name := range indexMapping.CustomAnalysis.SynonymSources {

the necessary APIs are there in the still open bleve PR
will open another PR updating the go.mod here later

[abhinavdangeti]

Gotcha. Let's not use v0.5.4 then. We'll set up a new version after the go.mod patching.

[abhinavdangeti]

#22