Add /r/tx/<txid>

[samedcildir]

This endpoint bypasses the index and acts as a proxy to get raw transaction hex from bitcoin cli. It returns hex encoded raw transaction data as a JSON string.

PR also includes recursion docs update to reflect the new endpoint and tests for the new endpoint.

[samedcildir]

@casey @raphjaph is there any progress about this PR? I guess we've gone through all scenarios about this endpoint in the Issue #4108 and we can use the endpoint with an immutable transaction parsing library. Thank you for sharing your insights and time.

[raphjaph]

LGTM