Use default ssi/ring in DID method crates

[clehner]

This updates DID method crates to follow the pattern of did:tz and did:webkey which use ssi/ring as a default feature, so that a build works by default.

e.g. these should all succeed:

cargo test -p did-ion
cargo test -p did-method-key
cargo test -p did-onion
cargo test -p did-pkh
cargo test -p did-tz
cargo test -p did-web
cargo test -p did-webkey

This should fix #169, #310, and #396.

DIDKit will need to be updated to set default-features=false for these crates - except for did-tz and did-webkey which already have this set.

A dev-dependency on did-method-key triggered default features on did-method-key; that dev-dependency but does not seem to be used anywhere, so is removed.