Support grpc-swift v2.0.0

[pkwarren]

In #1695, we've temporarily disabled updates to the grpc-swift plugin. The source code for the plugin is now found in a separate grpc-swift-protobuf project, which is versioned at v1.0.0.

We have a few options:

1. Continue tracking updates to grpc-swift but change the Docker build to ship grpc-swift-protobuf v1.0.0 artifacts. We'd have to manually track future updates and this wouldn't be great as the version of the plugin wouldn't match the version of the source used to build it. For example, if grpc-swift-protobuf bumped to v1.0.1 but grpc-swift stayed at v2.0.0, the best we could do is rebuild the docker image to push a new revision of grpc-swift.
2. Stop updates to current grpc-swift plugin. Create a new plugin to track grpc-swift-protobuf.

In either case, we'll need to update the Swift registry dependencies and the URLs for the project in buf.plugin.yaml.