For situations where the default network-based discovery is inappropriate
or inconvenient, this allows users to provide a hard-coded discovery
document for a particular hostname in the CLI config.

This is a new config block, rather than combined with the existing
"credentials" block, because credentials should ideally live in separate
files from other config so that they can be managed more carefully.
However, this new "host" block _is_ designed to have room for additional
host-specific configuration _other than_ credentials in future, which
might include TLS certificate overrides or other such things used during
the discovery step.