Introduce function to deserialize CloudConfig from Read

[RomanHargrave]

In many cases, it may be desirable to store the contents of the cloud configuration YAML outside of the local filesystem. In order to support this, all that should be necessary is to allow for CloudConfig to be deserialized from a Read, rather than a file located at some path, and to allow for the CloudSessionBuilder to be constructed from an existing CloudConfig.

I considered a CloudSessionBuilder constructor which accepted a Read, but settled on accepting CloudConfig instead as this seemed more logical by a small margin. I would ultimately be comfortable with either implementation, depending on maintainer preference.

Draft because I need to address some of the checklist items.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[github-actions]

cargo semver-checks found no API-breaking changes in this PR! 🎉🥳
Checked commit: 6b4bb35