Add our own version of c2pa "inception" actions to allow CAWG signers to specify provenance

C2PA's "inception" actions: `c2pa.created` and `c2pa.opened` are subject to rules which prevent their duplication in a manifest. If a tool/device is making use of a C2PA "minimal generator product", where the whole provenance of an asset is not being signed by the C2PA signer, we need the ability to specify our own inception actions.

For example, a tool creating a digital asset will want to specify that it has created the asset, but may then need to pass that action assertion to a a service which will sign the CAWG / gathered assertions with a C2PA certificate. That service will need to include an inception action, and if that service uses its own, `c2pa.opened`, to reflect that it was not involved in asset creation and the existing asset was passed to it, then we will need our own action types in a `cawg` namespace.

I don't think we'll need anything more than `cawg.created` and `cawg.opened` (or similarly worded), as the other C2PA action types do not have similar rules. As that changes, we may need to expand our action types.

Finally, I'm not sure if this lives in this particular repo / spec, as it is scoped to a metadata assertion, but it seemed like the most convenient place to track this issue.