Observability: pxf_stat_activity and pxf_cancel_backend

[ostinru]

Add pxf_stat_activity

Add pxf_stat_activity to help DBAs find out what happening inside PXF server.

postgres=# select * from pxf_stat_activity ;
-[ RECORD 1 ]-+---------------------------------------------------------
segment_id    | 1
session_id    | 57997
command_count | 8
xid           | 57997-0000000008
operation     | READ_BRIDGE
usename       | gpadmin
server        | ostinru-test
profile       | s3:parquet
schema_name   | public
table_name    | test_blob_read
data_source   | ostinru-test/topics/cuscom.email.created.v1/2026-05-29/*
query_start   | 2026-06-23 16:16:48.318+03
pxf_host      | example.com

Add pxf_cancel_backend and pxf_interrupt_backend

Add pxf_cancel_backend and pxf_interrupt_backend to allow killing running queries.

Update tests

1. Update pxfExtension tests - use testconatiners.
2. Explicitly define extension version. So, there is no need in sql file manipulations.

[ostinru]

Moved test-related code to separate PR #125 in order to keep PR focused.