Remove cosign dependency

[miki725]

currently chalk relies on cosign command for signing things. this is an external dependency which is outside of chalk control. ideally we should either use some library to achieve the same functionality or reimplement the core needs directly in nim

filing this ticket for tracking

currently there is some openssl code which is not compiled and its meant to be generating cosign keys:

in the cosign key material provider refactor will be removing it hence link here for future reference:

https://github.com/crashappsec/chalk/blob/4af0d69d9202008385cf1f000d0f6a86c5350c34/src/attestation.nim#L34-L98