feat(datasource/julia-pkg-server): add Julia `PkgServer`

[bauglir]

Changes

PkgServers are the primary package distribution system for Julia (the alternative being operating on the underlying Git repositories directly, which I might contribute later but seemed like a more daunting task given the number of possible hosting platforms). This datasource enables retrieval of packages from registries through these PkgServers. It supports multiple registries across PkgServers (a single server can host multiple registries and the same registry can be hosted across different PkgServers).

Julia-specific versioning is not yet implemented, I will implement that as a follow-up.

Context

• #6508.

Documentation

• [x] I have updated the documentation, or
• [ ] No documentation update is required

How I've tested my work

I have verified these changes via:

• [ ] Code inspection only, or
• [ ] Newly added/modified unit tests, or
• [ ] No unit tests but ran on a real repository, or
• [x] Both unit tests + ran on a real repository