Add MakeUCharSpan, to help constructing Span<[const] unsigned char>

Summary:
Partial backport (3/8) of core PR19326:
https://github.com/bitcoin/bitcoin/pull/19326/commits/e63dcc3a6752e7d406e7a650c2d6c2e95cd39aab

Depends on D9148.

Test Plan:

ninja

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9149