feat: Make CREATE EXTENSION tests optional

[mbiagetti]

The testing framework does not support conditional templating or conditional test execution, so the assertion part is generated but the generate-testing-values Dagger command.

Result of the generate command for

create_extension: true
dynamic_library_path: []
expected_extensions:
    - applied: true
      name: vector
extension_control_path: []
extension_image: registry.pg-extensions:5000/pgvector-testing:0.8.1-18-trixie
extensions:
    - name: vector
      version: 0.8.1
image_name: pgvector
ld_library_path: []
name: pgvector
pg_image: ghcr.io/cloudnative-pg/postgresql:18-minimal-trixie
shared_preload_libraries: []
sql_name: vector
version: 0.8.1

fixes #85

[mbiagetti]

I've removed it because when create_extension = false, the Database CR status doesn't have an extensions field at all - it's completely omitted by Kubernetes, not returned as an empty array. So asserting extensions: [] would fail. Let me know if I should dig in the testing framework to improve this part.