Fix OP-TEE client identity handling

[sangho2]

This PR fixes some bugs of OP-TEE shim's client identity handling. Default client identity is now Public not User. It also does some additional sanity checks.

[sangho2]

We should merge #865 first because this PR should have one more fix: per-session ClientIdenty rather than per-instance ClientIdentity. #865 heavily refactored related data structures and functions.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.47.0/src/lints/enum_variant_added.ron

Failed in:
  variant TeeLogin:ReeKernel in /home/runner/work/litebox/litebox/litebox_common_optee/src/lib.rs:850