Contribute SSH certificate signing solution

[milliams]

For Isambard, the Bristol team have developed an SSH certificate signing infrastructure which uses OIDC to authenticate. There is a server at Conch and a corresponding client at Clifton.

There will need to be work done to move the development into the OpenCHAMI infrastructure, generalise the code base to support a wide variety of solutions, integrate it into the quickstart (if appropriate) and document it in an externally-facing venue (e.g. a blog post).