Extend the tsig plugin to require TSIG signatures based on DNS opcodes,
similar to the existing qtype-based requirement.
The new require_opcode directive accepts opcode names (QUERY, IQUERY,
STATUS, NOTIFY, UPDATE) or the special values "all" and "none".
This is useful for requiring TSIG on dynamic update (UPDATE) or zone
transfer notification (NOTIFY) requests while allowing unsigned queries.
Example:
```
tsig {
secret key. NoTCJU+DMqFWywaPyxSijrDEA/eC3nK0xi3AMEZuPVk=
require_opcode UPDATE NOTIFY
}
```
Signed-off-by: Seena Fallah <seenafallah@gmail.com>
