New Extension API Proposal

API Namespace: [API Namespace Here]
API Owners: [Team Members, Team Aliases]
API Overview Doc: [Link to doc. Template is at https://docs.google.com/document/d/1mspntphE_vxwce4VNx08VtjsvgE9--ro3mg3lP1toeE/edit#]
Design Doc: [Link to design doc. This might be unnecessary with the overview doc above, in which case this can be N/A]
Supplementary Resources: [Any supplementary resources for your API or a larger feature that your API is a part of, such as PRDs, docs, mocks, etc.]