Azd tools/CLI Extension Framework

[rajeshkamal5050]

Adds the ability for partner teams to build and ship official extensions for azd.

Design Spec

### Tasks
- [ ] Define requirements for official extensions
- [ ] Partner team extension signing process

### Related Issues
- [ ] https://github.com/Azure/azure-dev/issues/4388
- [ ] https://github.com/Azure/azure-dev/issues/4390
- [ ] https://github.com/Azure/azure-dev/issues/4389
- [ ] https://github.com/Azure/azure-dev/issues/4391
- [ ] https://github.com/Azure/azure-dev/issues/4392