Add actions for reading metadata

[mike-marcacci]

We should add an action segment to all entities to "read metadata." Metadata would include viewing records (or at least version history), invocation logs, and potentially access logs in the future.

This might be introduced alongside #71 to support the feature in #72.

[mike-marcacci]

I propose changing the action segment from v2:

(basic).(details).(scopes).(secrets).(users)

to this in v3:

(basic).(metadata).(scopes).(secrets).(users)

Currently, details is conceptually identical to secrets but when applied to authorities and credentials. These two names could thus be consolidated into the more appropriately named secrets.

The new metadata segment (taking the second position from details, per the alphabetical convention used here) would only have provisions for an r as there is currently no mechanism for writing metadata.