Nargo should allow setting the active backend based on the directory name

[TomAFrench]

Problem

I would like to be able to select backends other than barretenberg to prove/verify against.

Happy Case

Nargo should examine the ~./.nargo/backends directory to find the list of backends which are available. Each backend shall exist in its own directory, i.e. barretenberg will live in ~./.nargo/backends/barretenberg. These names will currently be user specified when installing the backend (apart from barretenberg which will be installed by default).

Users however will not be expected to name the backend they want to use on every operation. We should instead maintain a small config file which will persist the desired backend, which the user can set in a nargo command.

Example flow

noirup

// Can get up and running with bberg immediately
nargo compile
nargo prove

// User creates a new directory with a backend  in `~/.nargo/backends/halo2`
// with a backend binary.

// Activate the new backend. 
nargo backend use halo2

// Can then carry on interacting with halo2
nargo prove
nargo verify

// Switch back to bberg
nargo backend use barretenberg

// Carry on interacting with bberg.
nargo prove
nargo verify

Alternatives Considered

Additional Context

No response

Would you like to submit a PR for this Issue?

Maybe

Support Needs

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