merge MetaSearch into QGIS core

cc @kalxas @rduivenvoorde @alexbruy @timlinux @jef-n  @m-kuhn 

This PR officially moves MetaSearch into QGIS core as per https://github.com/geopython/MetaSearch/issues/62.  Notes:

- workflow for out of band MetaSearch releases are TBD, but will be done from QGIS core
- there are existing translations in Transifex at:
 - https://www.transifex.com/projects/p/MetaSearch/resource/plugin_core/
 - https://www.transifex.com/projects/p/MetaSearch/resource/plugin_templates/
...which are not part of this PR.  Any ideas how to handle these?  n.b. docs have been ported to https://github.com/qgis/QGIS-Documentation, their Transifex translations need to ported as well

FYI this was originally submitted via #1711 but there were problems getting the tests to work.  This PR leaves out the tests which will be implemented later on.