Procedural api

[PietroPasotti]

this PR implements the procedural API described in #3

Fixes #3

This is how an interface test will look:

def test_no_data_on_joined():
    # nothing happens on joined: databags are empty
    t = Tester()
    state = t.run('openfga-relation-joined')
    t.assert_relation_data_empty()

where one MUST:

1. instantiate Tester within the scope of the test
2. call t.run
3. call one of
   a) t.assert_relation_data_empty, or
   b) t.assert_schema_valid or
   c) t.skip_schema_validation

in exactly this order. Otherwise the test will fail.

also, one CAN:

• write assertions on state, to verify it matches the expectations

tandem PR: canonical/charm-relation-interfaces#101

[gruyaume]

I mostly focused my review on user perspective and quality (and not so much on implementation) as I lack the requisite context for this project. Overall I think it looks good but we need better documentation.