Improve TLS identifier to include entire certificate chain

[tsandall]

Expected Behavior

Some users will want to have the entire certificate chain provided to the authz policy. One option would be to expose the entire certificate chain as the identity, however, this might be overly complex for most use cases. An alternative would be to let the identifier include multiple keys in addition to input.identity. In the x509 parsing built-ins we just JSON serialize the Go struct: https://github.com/open-policy-agent/opa/blob/master/topdown/crypto.go#L22. We could do the same here.

Actual Behavior

Today, the TLS identifier only extracts the subject.

[stale]

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[stale]

This issue has been automatically marked as inactive because it has not had any activity in the last 30 days.