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There is a boilerplate <https://github.com/pulumi/pulumi-provider-boilerplate>

Is the grpc protocol between the host and a provider also defined in a file somewhere, like protobuf, so that a provider can be implemented in e.g. Typescript?

Is it this one?
<https://github.com/pulumi/pulumi/blob/master/sdk/proto/provider.proto>

Yes - that's the one.

FWIW - There is an implementation of the RPC interface in TypeScript as part of the "Node.js dynamic provider" here: <https://github.com/pulumi/pulumi/blob/master/sdk/nodejs/cmd/dynamic-provider/index.ts>