Make repo config into the json config and support local file destination for artifacts

[sanfern]

L3afd is doesn't support downloading artifacts from multiple eBPF Package Repositories, enable l3afd to download from different package repositories by adding repo config into json config, specific to eBPF programs. Also allow local file destination for artifacts.

[dthaler]

@sanfern please add info for newcomers here

[sanfern]

Add an ebpf_program_repo_name attribute to the payload that will point to the eBPF package repository from where the artifact can be downloaded. If ebpf_program_repo_name begins with file:// then l3afd should search artifact in a local filesystem. If ebpf_program_repo_name attribute is not defined in the payload, it will get the kf-repo from l3afd.cfg file.