Supporting usage without full crev id

[kornelski]

Currently crev verify uses empty TrustSet when own id is not configured, but a completely empty set is not reporting anything. That makes a poor first impression.

Some trusted reviews could be easily added with a couple of crev trust …, but currently this also unhelpfully fails with "Current Id not set". Setting up an Id requires configuring a proof repo and a passphrase. That's a steep curve to run just one command.

So I'm looking into:

• Letting commands like verify run with non-empty ad-hoc trust sets. For example crev verify --trust https://proofs-repo could be useful in CI where you don't want to set up an identity, but want to automatically verify dependencies.

• Allowing gradual setup of a CrevId, so that they can be configured with trust information without configuring publishing.

• Having commands guide user to configure the missing parts as-needed, like adding trusted ids or configuring publishing URL.