feat(go/genkit): Remove indexers for localvec

[rutujaD1999]

Description here... Help the reviewer by:

• Issue : [Go] Remove Indexer as a primitive #2736
• Remove dependency on indexer functionality provided by genkit repo from the localvec plugin
• The indexer dependency is removed and a new helper indexer function is created in all the samples and unit tests to demonstrate an end to end flow.
• Modified the docStore and dbvalue types to be able to access externally for further using it in helper index functions
• Added getter and setter functions for docStore structure.
• The dependency on indexer functions from genkit will be removed from remaining plugins in separate PRs.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[apascal07]

Are you using this somewhere? doc-snippets is meant to be imported in docs and live docs aren't being read from here right now.