[chronik] add Electrum RPC server.donation_address

Summary:
See https://electrum-cash-protocol.readthedocs.io/en/latest/protocol-methods.html

Fulcrum sets a limit to 80 characters but allows any arbitrary string within that limit. Electrum ABC can handle wrong address formats via the usual "Pay to" URI parsing code.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17686