Remove the ReferenceExists requirement for <Add/Remove>MediaSigningCertificateAssignment

[bsriramprasad]

Addressing strong feedback from clients based on discussion here

• https://github.com/onvif/wg_addon_media-signing/issues/20

Discussion

• Remove requirement about throwing error when add or remove media signing assignment is called when streams are active with assigned key
• Instead keep the requirement similar to how its written for AudioSourceConfiguration or VideoSourceConfiguration

Proposed change

If a user-provisioned certification path is configured, affected streams may need to be restarted before the change takes effect.

This addresses the concern raised by Hans as well in other PR as the error requirement curtails the implementation behavior

• Clarify GetAssignedMediaSigningCertificates response behavior #739 (comment)