Provides DeploymentEnabled, DeploymentActiveAt, and DeploymentActiveAfter helpers for checking the status of buried deployments.
This is a partial backport of core#19438
https://github.com/bitcoin/bitcoin/pull/19438/commits/2b0d291da8f479739ff394dd92801da8c40b9f8e#
Note that `GetBlockScriptFlags` is off-by-one when applied to mempool transaction in the source material, hence the use of `DeploymentActiveAfter` instead of `DeploymentActiveAt` in our codebase (see explanation in D3361).
Depends on D12219