User transactions that request mutable access (requiring a write lock)
to the singleton `Clock` at address `0x6` will not be signed by honest
validators.

This prevents those transactions from (intentionally or otherwise)
bottlenecking throughput on transactions that reference the `Clock` by
taking an exclusive lock on it.

## Test Plan

New unit tests in `authority_tests`:

```
$ cargo nextest run -- test_invalid_mutable_clock_parameter
$ cargo nextest run -- test_valid_immutable_clock_parameter
```

## Stack

- #7922 
- #8054 
- #8055