Need support for 'client_credentials' in authenticate (for External Client Apps)

[dwindibank]

The nforce8 authenticate method doesn't support the client_credentials grant_type. Salesforce is requiring a migration from Connected Apps to External Client Apps. External Client Apps don't support username-password flow.

From: Comparison of Connected Apps and External Client Apps Features

3. External client apps use OAuth protocols to authorize third-party apps. However, there are some OAuth features that are available for connected apps but aren’t available for external client apps. For example, the OAuth username-password flow won't be made available for external client apps.

Adding the support isn't hard, just need to add an else to set client_credentials as the grant_type:

//TODO: Add JWT authentication
  if (opts.code) {
    bopts.grant_type = 'authorization_code';
    bopts.code = opts.code;
    bopts.redirect_uri = self.redirectUri;
  } else if (opts.assertion) {
    bopts.grant_type = 'assertion';
    bopts.assertion_type = 'urn:oasis:names:tc:SAML:2.0:profiles:SSO:browser';
    bopts.assertion = opts.assertion;
  } else if (opts.username || this.username) {
    bopts.grant_type = 'password';
    bopts.username = opts.username || this.getUsername();
    bopts.password = opts.password || this.getPassword();
    if (opts.securityToken || this.getSecurityToken()) {
      bopts.password += opts.securityToken || this.getSecurityToken();
    }
    if (this.mode === 'single') {
      this.setUsername(bopts.username);
      this.setPassword(bopts.password);
      this.setSecurityToken(bopts.securityToken);
    }
  } else {
    bopts.grant_type = 'client_credentials';
  }