Sort approver public keys in multisig

Consider making approver public keys in the multisig sorted, so the duplicates check in `assert_unique_approver_public_keys` becomes simpler (single loop instead of double loop). Afaict, there is no reliance on approver indices in the public key array, since each approver has the same weight.

Additionally, consider refactoring `AuthMultisigConfig::with_proc_thresholds` to take procedure thresholds as `BTreeMap` rather than `Vec` which avoids the explicit duplicate check and allows moving the setter threshold check from `AuthMultisig::new` to the config, to make invalid configs unrepresentable.

Context:
- https://github.com/0xMiden/protocol/pull/3246#discussion_r3569073672
- https://github.com/0xMiden/protocol/pull/3246#discussion_r3569102219