Make subscriptionId an optional parameter when looking up the endpoint service metadata

[phillipedwards]

Thank you for contributing! Before submitting your PR, can you please confirm that you have done the following?

• [ ] If you changed the values of the properties name, publisher, and galleryFlags in the vss-extension.json file, you have reverted them.
• [ ] If you changed the values of the properties id, name, author, you have reverted them.
• [ ] I have reverted any changes to the version number components in both vss-extension.json and task.json, OR I didn't change them.

• fixes: #122

[zbuchheit]

any updates on this issue?