salesforce: oauth support for api-only integration users

[aleksa-krolls]

Background

To date, we've only supported auth via username + password + token.
Salesforce integration users do not have a way to generate a security token, but rather authorize via Oauth and a Salesforce Connected App with api permissions-enabled.

See official docs: https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/intro_oauth_and_connected_apps.htm

Request

Add credential option that supports oauth for api-only users.

Users should still have their option to pick which credential type to use to authorize (either the current configuration schema or oauth).