Mark --cert option on run as is_eager

[bebleo]

Added is_eager=True to the --cert option on the CLI flask run command so that it is evaluated before --key. This allows the two options to be specified in either order.

• fixes flask run rejects --key foo.pem --cert foo.cert #4459

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.