Minor enhancements to TypeScript Types

[justsml]

• [x] Add provider names enumeration.
• [x] Limit OAuth versions from number to 1 | 2

[simov]

Thanks, @justsml.

However tslint seems to throw, either:

TS1336: An index signature parameter type cannot be a type alias.

or:

TS1337: An index signature parameter type cannot be a union type.

In the end you might be able to get it working, just note that I am trying to keep the type definitions as simple as possible.