Record 'contributing' Signers only in Certificate

[jpraynaud]

Issue

The signers field of the Certificate is recording all the eligible signers of the epoch which is confusing.
Instead, the signers field should include only the Signers that have contributed to creating the multi-signature (i.e. those who have sent valid individual signature(s))

Tasks

• Filter the signers and keep only the ones that have contributed when creating a certificate
• Reduce runtime interval of Mithril Signers hosted on GCP to 240s instead of 60s. This will avoid having always the same signers contributing (until we update the Protocol Parameters)