sql/sessions: immutable session variable

We should support setting an immutable session variable through a built-in. Once set the value of this session variable cannot be overridden. The built-in could be:
```
set_immut_session_var(varname, val);
```

This would be beneficial for Row Level Security where we prevent overriding a session variable within a session.

Jira issue: CRDB-49229

Epic CRDB-60943