Bug #78553 Convert MyISAM to an optional storage engine plugin

Description:
Convert MyISAM to an optional (but built and tested by default) storage engine plugin

- Remove its mandatory uses in the server, such as the last of MyISAM system tables.
- Make sure internal_tmp_disk_storage_engine accepts plugin SEs as args (or remove option altogether).
- Prepend myisam- prefix to all the MyISAM-specific options (e.g. key_buffer_size) so that they can be moved to the plugin from the server core.
- Make MyISAM build as a shared library plugin, ensure server works without it present, remove as many "if (table se is myisam) then this else that" checks from the core server as possible.
- Move MyISAM tests to a separate MTR suite.

The above list is probably incomplete.

How to repeat:
.

Suggested fix:
.

See also bug 78554

Posted by developer:
 
Reported version value updated to reflect release name change from 5.8 to 8.0

I think it makes sense to move MyISAM, MRG_MYISAM and FEDERATED out of the core and into a storage engine plugin.

See also:
Bug #82957 	Remove MRG_MYISAM (MERGE) storage engine
Bug #82959 	Remove FEDERATED storage engine

Then this plugin could/should be in a separate RPM package.

This also helps to improve and demonstrate the storage engine API etc.