feat: use redis connection string for cluster

[ohxyz]

Users can now use CACHE_REDIS_URL for RedisClusterCache. e.g.

cache = flask_caching.Cache(
    config={
        "CACHE_TYPE": "RedisClusterCache",
        "CACHE_REDIS_URL": REDIS_URL,
    }
)

• fixes Connect redis cluster with a redis url #551

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.