Test with automatic ssi branch selection

[clehner]

A common pattern is that functionality is added in https://github.com/spruceid/ssi/ which is then exposed in DIDKit - or ssi's API is changed which requires a corresponding update in DIDKit. Currently this involves updating the GitHub Action files in DIDKit to link to the appropriate commit of the ssi repo. e.g.: 06e017d
We could reduce the need for commit ref changes by automatically selecting the branch of ssi based on the branch in DIDKit, if a correspondence exists. For example, if didkit is checked out to branch feat/example-thing, the CI could check out ssi to feat/example-thing instead of main, if such a branch exists.