Introduce Membership TS type (take 2) (#4107)

* Introduce Membership TS type

* Adapt the Membership TS type to be an enum

* Add docstrings for KnownMembership and Membership

* Move Membership types into a separate file, exported from types.ts

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Co-authored-by: Stanislav Demydiuk <s.demydiuk@gmail.com>

src/types.ts

@@ -22,3 +22,4 @@ limitations under the License.
  */
 
 export type * from "./@types/media";
+export * from "./@types/membership";