Make `CREATE EXTENSION` tests optional

[gbartolini]

Right now, we always assume that the extension should be installed via the CREATE EXTENSION command. This was expected and part of the initial pilot.

As a result, the default tests always assume that we launch CREATE EXTENSION in the app database.

We should add a flag in the metadata file (e.g. create_extension, defaulted to true) to inform whether the extension supports CREATE EXTENSION or not. If not, we should make this block conditional in the Database definition:

  extensions:
  - name: ($values.sql_name)
    version: ($values.version)

And this block in the assert file:

  extensions:
  - applied: true
    name: ($values.sql_name)