Allow fixture scope to be customised

[markhobson]

Some live servers support fixtures with a scope other than session, for example pytest-flask. When using a narrower scope with a custom base_url fixture, it is unable to be injected into the wider scoped _verify_url fixture:

ScopeMismatch: You tried to access the package scoped fixture base_url with a session scoped request object, involved factories:
.venv/lib/python3.12/site-packages/pytest_base_url/plugin.py:19:  def _verify_url(request, base_url)

Could the fixtures support dynamic scopes to resolve this?

[BeyondEvil]

I'll welcome a PR that supports that!