Appflowy-indexer service and /api/search endpoint

[Horusiath]

This PR creates a new asynchronous service named appflowy-indexer which - in a similar fashion to appflowy-history - listens for incoming changes from redis streams collab events in order to decide if we should index or re-index appflowy documents for vector search purposes.

Later on these changes are available to for vector searching using HTTP GET over {appflowy-cloud-url}/api/search/{workspace}?query={query} endpoint in order to find documents matching searched phrase.

TODO list:

• [x] modify database a little - currently we only index plain text of the document, but we'll be doing other stuff like PDF and images as well, so we should prepare database accordingly.
• [x] fix appflowy-indexer docker issues (missing TLS certs)
• [ ] [in separate PR] publish OpenAI token usage to metrics.
• [ ] [in separate PR] figure out how to index existing documents without having users to open them first.

[appflowy]

LGTM