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

[matthewelwell]

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