Icinga DB: Config no_user_modify, Extract ValidateCertPath, and Support Redis username authentication

[oxzi]

Icinga DB: Config no_user_modify, Extract ValidateCertPath

Each configuration field of an IcingaDB Object was marked with no_user_modify as modifications via the API would not result in an actual change. While the Object would be updated, the internal Redis connection would not be restarted, resulting in an unexpected behavior.

The validation for cert_path and key_path was extracted from the generic Validate method to an own ValidateCertPath method.

The missing db_index was added to the documentation.

Icinga DB: Support Redis username authentication

The Redis ACL system was introduced with Redis 6.0. It introduced users with precisely granular permissions. This change allows Icinga 2 to use the Icinga DB feature against a Redis with an ACL user.

This was reflected in the documentation, next to the already implemented, but undocumented Redis database.

Closes #9536.

[oxzi]

Rebased after the last two CI fixes were merged.