add fmt formatter for `compaction_task_executor::state` and
`compaction_task_executor` and its derived classes.
this is a part of a series to migrating from `operator<<(ostream&, ..)`
based formatting to fmtlib based formatting. the goal here is to enable
fmtlib to print `compaction_task_executor`, its derived classes and
`compaction_task_executor::state` without the help of `operator<<`.
since all of the callers of 'operator<<' of these types now use
formatters, the operator<< are removed in this change. the helpers
like `to_string()` and `describe()` are removed as well, as it'd
be more consistent if we always use fmtlib for formatting instead
of inventing APIs with different names.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#14906
6c66030b7b