Add tests for some extensions currently not covered by the regression tests (#11191)

## Problem
Some extensions do not contain tests, which can be easily run on top of
docker-compose or staging.

## Summary of changes
Added the pg_regress based tests for `pg_tiktoken`, `pgx_ulid`, `pg_rag`
Now they will be run on top of docker-compose, but I intend to adopt
them to be run on top staging in the next PRs

docker-compose/ext-src/pgrag-src/sql/api_keys.sql

@@ -0,0 +1,16 @@
+-- API key function tests
+SELECT rag.anthropic_set_api_key('test_key');
+
+SELECT rag.anthropic_get_api_key();
+
+SELECT rag.openai_set_api_key('test_key');
+
+SELECT rag.openai_get_api_key();
+
+SELECT rag.fireworks_set_api_key('test_key');
+
+SELECT rag.fireworks_get_api_key();
+
+SELECT rag.voyageai_set_api_key('test_key');
+
+SELECT rag.voyageai_get_api_key();