Add support for VM gRPC services

[joshua-kim]

Why this should be merged

Adds native support for GRPC services being registered over the avalanchego http server. This adds support for streaming via grpc as an alternative to websockets.

How this works

1. Uses h2c's http handler to support plaintext http2
2. Makes a change to rpcchainvm to fix a bug where headers that are updated in the plugin process are not reflected on avalanchego's side by returning a response at the end of the Handle rpc that includes headers
3. Introduces a new rpc for the http request request.Body so that the plugin process can stream the body instead of the current behavior which uses io.ReadAll which may not be possible in the case of a streaming rpc.

How this was tested

Added a e2e test for unary + streaming rpcs over rpcchainvm

[joshua-kim]

grpc requires a header to indicate the end of an rpc, which is directly set onto the map in the plugin process (ref). We need to propagate this update back into avalanchego (I believe current rpcchainvm is not handling plugin-side header updates correctly).

[marun]

Would it be possible to elaborate as to the motivation for this addition? The current 'why this should be merged' in the description seems more like 'what'.

[marun]

(No action required) Will the stream be closed automatically on going out of scope (i.e. this explicit close is superfluous)? If not, maybe ginkgo.DeferCleanup is suggested?

[joshua-kim]

Would it be possible to elaborate as to the motivation for this addition? The current 'why this should be merged' in the description seems more like 'what'.

Added

[marun]

The upgrade failure is legit - I'm guessing you might need to rebase?

From the logs:

[09-04|15:26:54.904] FATAL chains/manager.go:392 error creating required chain {"subnetID": "11111111111111111111111111111111LpoYY", "chainID": "2BhsWVwQzJL67dSHQfkJ9hEgv79RPURVGeAzmmyeRkkX4tyn8A", "vmID": "mgj786NP7uDwBCcq6YwThhaN8FLyybkCa4zBWTQbNgmK6k9A6", "error": "error while creating new snowman vm database contains incompatible genesis (have ed752c73b66a9c010be62b2cec0088e9e120e6bdd7680fcec50bf880a0b13a87, new 751020f3cac7bf223d5754fef14810191c6669f758a2eb10be17dd31d43e81f2)"}

[joshua-kim]

The upgrade failure is legit - I'm guessing you might need to rebase?

I think my coreth branch fell behind master. Gonna rebase my coreth branch again.

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