QueryObservedTx and QueryTxSigners

[Version-unspecific]

This aims to improve tx/{hash} clarity as described here.

**Update:**
This MR (following feedback) similarly updates the display for tx/{hash}/signers as well, keeping a single ObservedTx OpenAPI schema. |
It also checks the OpenAPI TxSignersResponse fields against those of the new QueryTxSigners type, including !2688 (merged)'s commit (as though !2688 (merged) were already merged) in order to pass this test.