Add a helper which prints all prepared statements currently
present in the query processor.
Example output:
```
(gdb) scylla prepared-statements
(cql3::cql_statement*)(0x600003d71050): SELECT * FROM ks.ks WHERE pk = ?
(cql3::cql_statement*)(0x600003972b50): SELECT pk FROM ks.ks WHERE pk = ?
```
Closesscylladb/scylladb#26007