Support other package servers

[cmoesel]

An author on Zulip noted that they cannot access https://packages.fhir.org from their environment, but they have a mirror of the package server behind their firewall. They asked if we could add a configuration option to FSH Online to allow authors to specify their own package server.

See: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/FSH.20Online.20Configuration.20Request/near/394709662