feat(authn/oauth2): Add commands for byo oauth2

[awalker125]

This PR implements a BYO oauth config mechanism via halyard.

See

#956

spinnaker/spinnaker#2856

spinnaker/spinnaker#2664

Example byo comand

./hal config security authn oauth2 edit \
--client-id YYYYYYYYYYYYYYYYY \
--client-secret XXXXXX \
--provider byo \
--access-token-uri https://example.com/login/oauth/access_token \
--user-authorization-uri https://example.com/login/oauth/authorize \
--user-info-uri https://example.com/api/v3/user \
--scope all \
--client-authentication-scheme query \
--user-info-requirements foo=bar \
--user-info-requirements baz=qux \
--user-info-mapping-username username \
--user-info-mapping-first-name firstname \
--user-info-mapping-last-name lastname \
--user-info-mapping-email email```

[stewchen]

Nice!

[usrleon]

Current documentation says I must provide all data in gate-local.yml file, but according to example all data provided in command line. So what is the way?

[awalker125]

@usrleon currently it must be done in gate-local.yml. If this gets approved then either should work.

[stewchen]

We will move from setting gate-local.yml to using halyard like described here once the PR is merged and released.

[ttomsu]

Can we name this "other" or "custom"? Simply "byo" isn't clear that it means "bring your own".

[awalker125]

I've changed byo to other