LocalTxMonitor: Add support for GetMeasures

[fraser-iohk]

Description

This PR adds a new message to LocalTxMonitor, GetMeasures, which is similar to GetSize (returning the current mempool transaction count, the size of those transactions, and the total mempool capacity), but returns information about several other metrics in addition to the byte size and capacity of the mempool. GetMeasures currently returns size and capacity information for transaction byte size, ledger's execution units (both their memory and execution steps), and the reference scripts size, but can easily be extended to support any new metric types in future while remaining backwards compatible.

This change works towards ouroboros-consensus#1178, and has a dependent PR in ouroboros-consensus

Checklist

Quality

• Commit sequence makes sense and have useful messages, see ref.
• New tests are added and existing tests are updated.
• Self-reviewed the PR.

Maintenance

• Linked an issue or added the PR to the current sprint of ouroboros-network project.
• Added labels.
• Updated changelog files.
• The documentation has been properly updated, see ref.