feat: compare this with ctx.subject (#1204)

[cmmoran]

In the conversation for #1204, @hperl commented:

From an implementation point of view, this would require:

• adding support for this expression in the OPL parser
• adding a new userset rewrites type (maybe SubjectEqualsObject), and attach it to the relation if we parsed the expression
• implement the SubjectEqualsObject rewrite in the check engine (simple, just compare object and subject for a given relation tuple)

Of course, add tests to make sure your change was effective, and run the fuzzers if you touch parser code.

This PR addresses the above suggested implementation and adds the related feature.

Related issue(s)

#1204

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
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  the changes.
• I have added updated tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

I will amend this PR to update/add to the documentation regarding this new feature.