chore: standardize pg client naming on Pg casing with deprecated aliase

[karenc-bq]

Summary

Standardizes the PostgreSQL module on the dominant Pg casing.
This is a non-breaking change. The old PG-cased names are marked as deprecated and we export backwards-compatible aliases.

Description

Changes

Public API:

• AwsPGClient → AwsPgClient
• AwsPGPooledConnection → AwsPgPooledConnection
• The former names remain exported as @deprecated aliases that resolve to the new names (both value and type), so new AwsPGClient(...), extends AwsPGClient, instanceof AwsPGClient, and AwsPGClient-typed annotations all keep working.

Internal:

• PGClient → PgClient, PGPoolClient → PgPoolClient interfaces
• Local variables (awsPGPooledConnection → awsPgPooledConnection)

Docs, examples, and benchmarks:

• Migrated all customer-facing docs (docs/using-the-nodejs-wrapper/**), example projects, and benchmark scripts to the new names.
• Added a deprecation callout in the client usage guide and a CHANGELOG entry announcing the rename + deprecation.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.