Load backends dynamically

[kevaundray]

Problem

Currently backends are pulled in at compile time, which means we cannot use the same strategy as rustup which loads backends and switches them out at runtime.

Solution

Dynamically load libraries

Alternatives considered

(Describe any alternative solutions you have considered.)

Additional context

(If applicable.)

[kevaundray]

This begs the question as to whether we need a Proving system trait in acvm or a c header that specifies the methods needed

[kobyhallx]

Dynamically load libraries requires number of considerations.
Those to be have once dynamic backend invocation gets completed.