Merge #13424: Consistently validate txid / blockhash length and encoding in rpc calls

Summary:
5eb20f81d9 Consistently use ParseHashV to validate hash inputs in rpc (Ben Woosley)

Pull request description:

ParseHashV validates the length and encoding of the string and throws
an informative RPC error on failure, which is as good or better than
these alternative calls.

Note I switched ParseHashV to check string length first, because
IsHex tests that the length is even, and an error like:
"must be of length 64 (not 63, for X)" is much more informative than
"must be hexadecimal string (not X)" in that case.

Split from #13420

Tree-SHA512: f0786b41c0d7793ff76e4b2bb35547873070bbf7561d510029e8edb93f59176277efcd4d183b3185532ea69fc0bbbf3dbe9e19362e8017007ae9d51266cd78ae

Backport of Core PR13424
https://github.com/bitcoin/bitcoin/pull/13424/

Depends on D5292

Test Plan:

ninja check
ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5294