cli: simplify the configuration of logging

[knz]

Today we have two flags --log-dir and --sql-audit-dir that configure the main logging directory and the SQL audit log directory separately.

Ideally, we want configurability per logger.

Also if we start supporting network loggers we'll want that to be configurable too.

To avoid a proliferation of flags, we should migrate away from discrete flags and into a more structured configuration format, akin to what is done already with --store.

We need to answer the following questions:

• do we want to support bundling? (multiple loggers to the same sink)
• do we want to support teeing? (single logger to multiple sinks)

With the status quo (bundling only possible to the same directory, i.e. no sink bundling, and no teeing) we could have:

--log-output=<channel>:<sinktype>:<parameters/path>

to configure a sink/path per logger; or conversely

--log-output=<sinktype>:<channel>:<parameters/path>

Special cases:

• without a configuration, we'd have a default derived from the first non-memory --store, as currently
• if --log-dir / --sql-audit-dir are specified, they would be used as defaults too
• each logger without a configured sink defaults to the same parameters as the "general" logger.

[knz]

Discussed in #51990

[knz]

Fixed by #57134