Version the extension contract and lock down GA.

Now that we've GA'ed extensions, I think we should lock down the contract and start versioning our `.proto` files by doing something like:

1. Move the current `.proto` files to somewhere like `/extensions/proto/v1` so they're easy to discover. We could also maybe consider `/schemas/extensions/proto/v1` instead of we wanted all of our contracts to live in one spot.
2. Change the name of the package in those `.proto` files from `azdext` to something more like `azd.extensions.v1`.
3. Copy `v1` to a sibling `v2beta` and change the package names to `azd.extensions.v2`.
4. Remove `telemetry.proto` from `v1` since it was added last night and should only exist in `v2beta` for now.
5. Add a linting rule that runs `buf breaking` to make sure `v2beta` only adds to `v1`. Maybe consider a `CODEOWNERS` line that requires explicit approval from a handful of people if you touch anything in the `v1` folder since that's now GA.
6. Change `azdext` to generate off `v2beta`.

If people really hate moving the `.proto` files, then I think we can just do the `v1` / `v2beta` versioning in the current `/cli/azd/grpc/proto` folder. We should also double check that's the right way to version GRPC because I've never gone deep on the sharp edges there.