Improve str(OAuthTokenResponse)

[sirosen]

This makes the str() of an oauth response object more verbose, clarifying that by_resource_server is a field (not the set of top-level keys) and including mention of id_token. The output is also now more YAML-like, and not just an undecorated JSON dump.

In practice, the id_token can be very long, so this implementation truncates it to the first 10 chars and notes it as ... (truncated).

No other fields are added to the stringifier as of yet.

resolves #639

---

Example output (with tokens redacted):

OAuthTokenResponse:
  id_token: eyJhbGciOi... (truncated)
  by_resource_server:
    {
      "auth.globus.org": {
        "scope": "openid email profile",
        "access_token": "REDACTED",
        "refresh_token": null,
        "token_type": "Bearer",
        "expires_at_seconds": 1667661839,
        "resource_server": "auth.globus.org"
      },
      "transfer.api.globus.org": {
        "scope": "urn:globus:auth:scope:transfer.api.globus.org:all",
        "access_token": "REDACTED",
        "refresh_token": null,
        "token_type": "Bearer",
        "expires_at_seconds": 1667661839,
        "resource_server": "transfer.api.globus.org"
      }
    }