System Table version of CHANGE MASTER

CHANGE MASTER options have diversified enough that making them a System Table - or at least implemented with SQL table infrastructures - can unify ecosystems and improve flexibility, both user-side and in code.

Here are some features that could be valuable for the CHANGE MASTER command, and SQL tables already have their infrastructures:

• Null/default values → MDEV-28302
• Support for auto-conversion and SQL expressions as values
• Distinct error for unrecognized options (compared to "SQL syntax error")
• Batch configuration of multiple connections (tables use DML such as UPDATE)
  • The current finest precision is MDEV-28302's global defaults' batching of every connection with no per-connection override.
• EXPLAIN integration for listing expected data types in detail (including precision and NULLability), no documentation required
• Reading (tables use DQL, i.e., SELECT)
  • The current method, information_schem.slave_status
    • Has some columns using different names than their CHANGE MASTER counterparts
    • Shows the effective value (e.g., resolved MDEV-28302 DEFAULT) by design, but not whether an option was left unset.
    • Includes statistics in addition to CHANGE MASTER configurations

A note on the storage engine choice:
While Aria is the System Tables' go-to default, a pseudo-engine (as in a writable view) based on MDEV-36285 or MDEV-31342/MDEV-31535 may fit CHANGE MASTER better.

• If the configurations are stored in Aria's datafiles rather than @@master_info_file & relay_log_info_file, then those files become a second-class feature kept only for upgrade/downgrade compatibility.
• Having the data in storage engines' binary format also hinders access in code.
  (In practice, Replication itself should be a bigger user of this table than DBAs.)
• It was already a long-time known limitation that @@master_/relay_log_info_file are not ACID-level crash safe. (MDEV-8946)
• do_domain_id and ignore_domain_id are always mutually exclusive, and master_log_file/pos only apply when master_use_gtid=NO.