[SharePoint] Extend Metadata Indexing to custom one

Issue created in another repo, replicated here for visibility: https://github.com/elastic/connectors-ruby/issues/499

### Description

In https://github.com/elastic/connectors-python/issues/1268, we removed many metadata fields from what we index for Sharepoint Online. We did this by adding explicit `$select` clauses to our queries to the Graph API, which tells the API which fields to send in the response.

However, we anticipate that many customers will want fine-grained control over what fields they fetch and index, but will also not want to make code changes in order to fetch more/different fields.

Start with the linked connectors-python issue 1268 and the replicated connectors-ruby issue 499 to understand the existing SharePoint Online metadata selection. Review how Graph API $select clauses currently control fetched fields, then define what customer-configurable field selection should look like and how completion will be verified.

Written by the indexing model from the issue text.