deploymentinfo: Add DeploymentName()

Summary:
Add and use DeploymentName(). Add missing includes in blockchain.cpp. Rename BIP9SoftForkDescPushBack -> SoftForkDescPushBack. Add a SoftForkDescPushBack function for buried deployments (currently not used in the RPC)

This is a partial backport of core#19438
https://github.com/bitcoin/bitcoin/pull/19438/commits/92f48f360da5f425428b761219301f509826bec4
https://github.com/bitcoin/bitcoin/pull/19438/commits/8ee3e0bed5bf2cd3c7a68ca6ba6c65f7b9a72cca

Notes:

• D5218 already replaced the name argument with a DeploymentPos in BIP9SoftForkDescPushBack
• we don't report buried deployment in getblockchaininfo, so this backport only maintains the current code we have for active BIP9 activations. A DeploymentName function is added for buried deployments, but currently unused.

Depends on D12222

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12224