Added event for media signing key status

[bsriramprasad]

Condition	Operation	Event Source (KeyID)	Effective	Clarification
No signed streams using the manufacturer-provisioned key are active	AddMediaSigningCertificateAssignment with a user-provisioned key	User-provisioned key	True	The user-provisioned key becomes immediately effective for newly established signed streams.
Signed streams using the manufacturer-provisioned key are active	AddMediaSigningCertificateAssignment with a user-provisioned key	User-provisioned key	False	Existing signed streams shall continue using the manufacturer-provisioned key until terminated. The user-provisioned key is not yet effective for newly established signed streams.
No signed streams using the user-provisioned key are active	RemoveMediaSigningCertificateAssignment	Manufacturer-provisioned key	True	The manufacturer-provisioned key becomes immediately effective for newly established signed streams.
Signed streams using the user-provisioned key are active	RemoveMediaSigningCertificateAssignment	Manufacturer-provisioned key	False	Existing signed streams shall continue using the user-provisioned key until terminated. The manufacturer-provisioned key is not yet effective for newly established signed streams.

Behavioral Clarification

• Effective=True indicates that the event source key is effective for newly established signed media streams.
• Effective=False indicates that the event source key has been selected but is not yet effective because existing signed streams are still using the previously effective key.
• A key transition becomes effective only when no signed streams using the previously effective key remain active.
• At any point in time, only one key shall be effective for newly established signed media streams.

[bsriramprasad]

Abandoning this PR towards adding a clarification in #777