rpc: create rpc/mining.h, hoist default max tries values to constant

Summary:
This is a backport of core#19133 [1/10]
https://github.com/bitcoin/bitcoin/pull/19133/commits/cb00510dbac99b44f3f2cf6e58bb2e4401c5ef28

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9941