[Chronik] Refactor: Add HashOrHeight enum

Summary: Factors out the parsing of a string that's either a hash or height, so we can use it elsewhere.

Test Plan: ninja check-functional

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13857