Support for enterprise use with all http calls through internal mirrors/caches

### Is your feature request related to a problem? Please describe

It would be really great to support internal mirroring of the asdf plugins as well as the download of the tools so that an enterprise wishing to build things could not make any external requests beyond the mirrors.

Some of the ideas would be

1. configuration - .asdfrc? some global location?
2. mirroring - the ability to mirror all or parts of the plugins and tools
3. ensuring that every download or query examines the mirror

### Describe the proposed solution

1. Add configuration to `~/.asdfrc` or possibly a global location as well (when using docker images, would be nice to configure all access regardless of the user) that configures locations of mirror or mirrors for various pieces - plugins, tools
2. Ensure that mirroring is possible and documented for an organization to either mirror regularly or "passthrough cache"

### Describe similar `asdf` features and why they are not sufficient

I don't see similar features

### Describe other workarounds you've considered

Possibly setting up a proxy of some sort could cache - like a squid proxy, but would require forcing asdf to use it.

Start by reviewing the proposed ~/.asdfrc and possible global configuration locations, including the Docker-image use case. Map the requested plugin and tool mirror behavior and identify how downloads and queries would need to honor it; done should include documented enterprise setup and no external requests beyond configured mirrors.

Written by the indexing model from the issue text.