[Code Embeddings] Handle force pushes

Context

From gitlab-elasticsearch-indexer!698 (comment 2678313150):

If we can detect force-push scenarios there, we could pass a flag to force re-indexing (like setting fromSHA = "") instead of the current tree-diff approach.

References

Further context in related issue: gitlab-elasticsearch-indexer#172 (closed)

Proposal

We can make use of a reindexing field in the documents such that:

• Add a reindexing field to indicate a document is in the process of reindexing

  • issue: Add reindexing column in ActiveContext Code (!201428 - merged)

• In the Go Indexer, make use of the reindexing field to perform a reindex if ran with the options from_sha="" and force_reindex=true

  • issue: [Code indexing pipeline] Handle force_reindex (gitlab-elasticsearch-indexer#172 - closed)

• In Rails, when there is a force-push, call the Go Indexer with options from_sha="" and force_reindex=true (this issue)