Use same defineNewContext method for coprocessor signers

[jatZama]

in InputVerifier contract, as the one already existing for KMS signers in KMSVerifier, with a customizable threshold, instead of addSigner and removeSigner functions.
Similarly we should add a setThreshold as well in InputVerifier contract.
Another question is should we rename the defineNewContext methods to something more explicit , for eg defineNewCoprocessorContext and defineNewKMSContext ? same question for the setThreshold method? wdyt @jot2re @RomanBredehoft @dd23

[dd23]

Related Slack discussion: https://zama-ai.slack.com/archives/C0320EL7Y4U/p1747037453826069

Yes, I think it would be good to be explicit and rename to defineNewCoprocessorContext and defineNewKMSContext and the same for setThreshold.

[melanciani]

by the way I think it makes sense to simplify the name and go with :

• addKmsContext
• addCoprocessorContext

wdyt ? I was going with these names in the gateway but I believe we should align on the names here

cc @PacificYield