Add a juju action to show all reldata in StoredState

[mthaddon]

Enhancement Proposal

When setting the environment variable for a particular workload, it's useful to know what variables are available to you, which are defined in reldata within StoredState.

The charm should have an action to show all the data so someone can more easily determine what to use in the environment variable.

This action should be called show-environment-context. We should also mention this in the description of the environment config option.