add(consensus): Add a target_difficulty_limit field on testnet::Parameters

[arya2]

Motivation

We want to use the same target_difficulty_limit on Regtest as zcashd and allow for it to be configured on custom testnets.

Closes #8479.

PR Author Checklist

Check before marking the PR as ready for review:

• Will the PR name make sense to users?
• Does the PR have a priority label?
• Have you added or updated tests?
• Is the documentation up to date?

For significant changes:

• Is there a summary in the CHANGELOG?
• Can these changes be split into multiple PRs?

If a checkbox isn't relevant to the PR, mark it as done.

Solution

• Adds a target_difficulty_limit field on testnet::Parameters
• Returns it for Testnets in the Network::target_difficulty_limit method
• Checks block difficulty thresholds in block/checkpoint verifiers

Testing

Manually tested that the internal miner still works on Regtest, and now that the block/checkpoint verifiers validate the difficulty threshold, the changes in this PR are covered by the validate_regtest_genesis_block and regtest_submit_blocks acceptance tests.

Review

Anyone can review.

Reviewer Checklist

Check before approving the PR:

• Does the PR scope match the ticket?
• Are there enough tests to make sure it works? Do the tests cover the PR motivation?
• Are all the PR blockers dealt with?
  PR blockers can be dealt with in new tickets or PRs.

And check the PR Author checklist is complete.