catalog: Allow for more builtin roles

[jkosh44]

This commit updates role ids to have two separate namespaces. One for builtin roles and one for user roles. This will allow us to more easily add builtin roles in the future.

This is a breaking change.

Motivation

This PR refactors existing code.

Checklist

• [X] This PR has adequate test coverage / QA involvement has been duly considered.

• [x] This PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way) and therefore is tagged with a T-protobuf label.

• [X] This PR includes the following user-facing behavior changes:

  • Changes role ids to be a string, which changes the id column of mz_roles.

[benesch]

Skimmed the test changes and I love it. Deferring the actual review to @mjibson though.

[jkosh44]

@mjibson In case you already started reviewing, I just changed the name of the added user from "mz_system_external" to "http_default_user" because I thought the old name was really non-descriptive.

[jkosh44]

@mjibson In case you already started reviewing, I just changed the name of the added user from "mz_system_external" to "http_default_user" because I thought the old name was really non-descriptive.

Oops didn't follow my own rule and gave it a bad name without a "mz_" prefix. Just fixed the name of the role.