refactor: move CBlockIndex#ToString() from header to implementation

Summary:
which allows dropping tinyformat.h from the header file.

This concludes backport of core#25349
https://github.com/bitcoin/bitcoin/pull/25349/commits/3a61fc56a0ad6ed58570350dcfd9ed2d10239b48

Depends on D16072

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16073