Merge #14982: rpc: Add getrpcinfo command

Summary:
PR14982 backport.

a0ac15459a0df598e1ee1fd36a3899a129cecaeb doc: Add getrpcinfo release notes (João Barbosa)
251a91c1bf245b3674c2612149382a0f1e18dc98 qa: Add tests for getrpcinfo (João Barbosa)
d0730f5ce475e5a84da7c61fe79bcd6ed24d693e rpc: Add getrpcinfo command (João Barbosa)
068a8fc05f8dbec198bdc3fe46f955d8a5255303 rpc: Track active commands (João Barbosa)
bf4383277d6761cc5b7a91975752c08df829af72 rpc: Remove unused PreCommand signal (João Barbosa)

Pull request description:

The new `getrpcinfo` command exposes details of the RPC interface. The details can be configuration properties or runtime values/stats.

This can be particular useful to coordinate concurrent functional tests (see #14958 from where this was extracted).

Backport note: includes some boilerplate changes to getrpcinfo()
code (we don't have RPCHelpMan yet).

Depends on D5172

Test Plan: ninja check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5173