Support grpc-web with JSON

[kjvalencik]

Feature Request

Add support for application/grpc-web+json which is identical to grpc-web, but uses the canonical JSON representation instead of protobuf.

Crates

• tonic-web
• tonic

Motivation

grpc-web+json is advantageous for browser based clients because data is serialized in a format natively understood by JavaScript (JSON). Eliminating protobuf serialization can reduce library dependency and significantly reduce package sizing while also providing performance benefits.

Proposal

Detect Content-Type: application/grpc-web+json and switch to the Json codec in tonic-web.

Alternatives

This may be especially difficult because, currently, supporting both grpc and grpc+json requires building twice and switching codec implementations based on headers. See #851.

As an alternative, tonic-web could be updated to be agnostic of the content type and applications could switch services based on content type similar to workaround for grpc+json.