This PR introduces the concept of subject _types_ to be associated with each subject in the `SubjectMetadataController`, and used for control flow in our RPC stack (`setupProviderEngine` and so forth).
We already differentiate between "types" of subjects in various places on an ad hoc basis via boolean flags (e.g. `isInternal` in our RPC stack) or the presence/absence of certain values in the subject's metadata (specifically `metadata.extensionId`). The status quo is manageable if not ideal, but will start to become untenable with the introduction of Snaps in the near future.
Therefore, this PR establishes a `SUBJECT_TYPES` enum and adds the `subjectType` property to the metadata of each subject. A new migration is added to accomplish this. Finally, we specify and `INTERNAL` subject type to distinguish internal from external requests.
