rpc: Limit echo to 10 args

Summary:
This is a backport of core#19528 [3/4]
https://github.com/bitcoin/bitcoin/pull/19528/commits/fa50bdc755489b2e291ea5ba0e39e44a20c6c6de

Note: one line of documentation for the echo RPC seems to have been
accidentaly deleted in D5920. This restores it before applying the
change.

Depends on D10081

Test Plan: ninja && ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10082