feat: add libpq interface for encrypted and non-blocking cancellation

feat: add libpq interface for encrypted and non-blocking cancellation

Encrypted and non-blocking cancellation should be part PostgreSQL 17:
- https://commitfest.postgresql.org/37/3511/
- https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=61461a300c1cb5d53955ecd792ad0ce75a104736

We here add the interface for libpq functions for this feature.

An extra test is introduced, it closely reproduces
src/test/modules/libpq_pipeline/libpq_pipeline::test_cancel() in
PostgreSQL test suite.

The error_message property of PGcancelConn is directly an str and
PGcancelConn has no encoding specified and all messages come from the
client (libpq).