feat: make pytest-base-url an optional dependency

[Pavel Feldman (pavelfeldman)]

pytest-base-url (MPL-2.0) and its transitive requests/certifi dependencies get flagged by license scanners. It's now an optional extra, pip install pytest-playwright[base-url].

When pytest-base-url is not in use, the plugin itself provides --base-url (defaulting to $PYTEST_BASE_URL), the base_url ini option and the base_url fixture. Only --verify-base-url needs the extra. Whether pytest-base-url is in use is decided in pytest_load_initial_conftests, once every plugin from -p, entry points and the initial conftests is registered, and the options are added there (the command line is parsed afterwards). This covers an installed but not loaded plugin (PYTEST_DISABLE_PLUGIN_AUTOLOAD=1), -p no:base_url, and -p base_url or -p pytest_base_url.plugin in any order relative to this plugin. Environments that already have pytest-base-url installed behave as before.

Tests: --base-url, the ini option, the env var, and no value at all, each with pytest-base-url disabled; and plugin autoload disabled with and without pytest-base-url loaded explicitly (sync + async). A new CI job uninstalls pytest-base-url and runs the base-url tests, so the "not installed" path is covered as well. A fresh install no longer pulls in pytest-base-url, requests or certifi.

Release note: anyone relying on --verify-base-url needs pytest-playwright[base-url]. Docs follow-up: the "Configure base-url" section on playwright.dev should mention the extra.

Fixes #311

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