pybind/mgr: add sqlite3 db module helpers

pybind/mgr: add sqlite3 db module helpers

This creates a new '.mgr' pool for storing a default sqlite3 database
for each mgr module. Each module's database is stored in:

        file:///.mgr:<mgr module name>/main.db?vfs=ceph

The "main.db" is the only one used presently but perhaps a module may
want extra databases for some reason. The module name is used for the
RADOS namespace.

Databases are versioned in a common table called MgrModuleKV using the
"__version" key. A mechanism is in place (SCHEMA_VERSIONED) to allow
modules to upgrade their databases over time in a consistent way.

Fixes: https://tracker.ceph.com/issues/50278
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>