Integrate Cosign to sign Ballerina Docker Images

[Miranlfk]

Description:
Integrating Sigstore's Cosign to sign the Ballerina Docker Image. Cosign can be used to sign and verify the docker images, in which this case we would use this tool for the Ballerina Image. This would further provide user's the confidence that Ballerina is improving the security in its supply chain.

Describe your problem(s)
Ballerina may be vulnerable to supply chain attacks. If a release of Ballerina is compromised, and its passed on to the user, this will negatively affect his/her experience and thereby hinder the adoption of Ballerina.

Describe your solution(s)
Use the Cosign tool along with the other tools from Sigstore to sign the images being released, the user can thereafter verify it on his/her end.