For principal - subordinate plugin type relations where the
principal Python payload imports code from packages managed by a
subordinate, upgrades can be problematic.
This change will allow a subordinate charm that have opted into the
feature to inform its principal about all implemented release -
packages combinations ahead of time. With this information in place
the principal can do the upgrade in one operation without risk of
charm relation RPC type processing at a critical moment.
Also sync c-h.
Closes-Bug: #1806111
Change-Id: I95567d5d047eb64842436e671b74a633e6f509f4
e9fc1de43b