Support nested objects in credentials with a JSON field type

We have a use-case where the http adaptor wants to send a nested JSON obejct as part of a credential.

Usually credentials are flat, like this:

{
  user: 'admin',
  password: 'password'
}

But in HTTP we want to use a JSON object as the value for a complex field. Like this:

{
  user: 'admin',
  password: 'password',
 tls: {
     ca: '--CERT--'
  }
}

The lightning UI can't really handle this as it stands - it assumes that credentials are simple primitive values in the JSON schema.

What I suggest is that we allow any complex fields to accept raw JSON as a value. Then users can specify objects of arbitrary complexity as plain JSON

Start by tracing how the Lightning UI represents credential fields from the JSON schema, then follow the HTTP adaptor path that consumes those credentials. Check how primitive and complex fields are currently handled, and define completion as accepting nested JSON objects of arbitrary complexity while preserving the existing flat credential behavior.