refactor: move *index constants out of validation

Summary:

• move DEFAULT_TXINDEX from validation to txindex
• move DEFAULT_COINSTATSINDEX from validation to coinstatsindex
• move DEFAULT_BLOCKFILTERINDEX from val to blockfilterindex

This is a backport of core#26235

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15795