Allows users to specify virtual environment for dependencies

[psschwei]

What is the expected enhancement?

We currently allow users to specify a list of dependencies to pass to their program: https://github.com/Qiskit-Extensions/quantum-serverless/blob/841db743ff3334fa95e277c53093e30bd0a441d7/gateway/api/ray.py#L104

We should consider allowing users to alternatively specify a conda environment (using the conda parameter for runtime_env) as opposed to a list of dependencies to install via pip.

This would be useful in particular for HPC environments running on non-standard architectures (where installing via pip can be tricky).