mining: add new getCoinbaseTx() returning a struct

Introduce a new method intended to replace getCoinbaseRawTx(), which
provides a struct with everything clients need to construct a coinbase.
This is safer than providing a raw dummy coinbase that clients then have
to manipulate.

The CoinbaseTx data is populated during the dummy transaction generation
and stored in struct CBlockTemplate.

Expand the interface_ipc.py functional test to document its usage
and ensure equivalence.

doc/release-notes-33819.md

@@ -1,5 +1,8 @@
 Mining IPC
 ----------
 
-- The `getCoinbaseTx()` method is renamed to `getCoinbaseRawTx()`.
+- The `getCoinbaseTx()` method is renamed to `getCoinbaseRawTx()` and deprecated.
   IPC clients do not use the function name, so they're not affected. (#33819)
+- Adds `getCoinbaseTx()` which clients should use instead of `getCoinbaseRawTx()`. It
+  contains all fields required to construct a coinbase transaction, and omits the
+  dummy output which Bitcoin Core uses internally. (#33819)