feat: Add convenience APIs

[tmathern]

Code changes:
Some of our other SDKs have convenience methods that help dealing with manifests.
I thought this is a good idea to have too for developer experience, so I added the following, all on the Reader:

• get_active_manifest, returns the active manifest (parsed as a dictionary for easy use);
• get_manifest_by_label, retrieves any manifest based on its label/id and returns it as dictionary;
• get_validation_state, returns just the validation state of the whole manifest store;
• get_validation_results returns all the validation results from the manifest store (and only the validation results).

Workflow changes:

• Add formatting and syntax check in github workflow