Filter flags / identities endpoints to only return flags with a given tag

### Is your feature request related to a problem? Please describe.

From a customer request. They are currently retrieving the features using an endpoint on the Core API to map features and tags, then filtering out flags based on the relevant tags.

### Describe the solution you'd like.

Ability to provide an optional (list of) tag(s) to filter the flags when retrieving the flags for the environment or for an identity.

### Describe alternatives you've considered

N/a

### Additional context

_No response_