Expose backend identifier information

[TomAFrench]

Problem

We currently have no way for nargo to actually know what backend it's interacting with. An example of where this is useful is embedding this into preprocessed artifacts as these are backend specific so it's important to know which backend to use.

Proposed solution

We should add a function which exposes useful information for callers. This could be as simple as exposing the backend identifier but we may want to expose more information such as turbo vs ultraplonk, etc.

Alternatives considered

No response

Additional context

No response

Submission Checklist

• Once I hit submit, I will assign this issue to the Project Board with the appropriate tags.