Support HTTP/JSON via gRPC Gateway

[jzelinskie]

This change adds an additional port that the spicedb serve command uses to serve an HTTP/JSON API.
It uses gRPC Gateway to forward requests to the --grpc-addr. Each RPC is represented as an HTTP POST with the protobuf messages encoded in JSON in the body of the request.

Example usage:

$ spicedb serve --grpc-preshared-key "somerandomkeyhere" --grpc-no-tls --http-no-tls --log-level debug &
$ curl -X POST -H "Authorization: Bearer somerandomkeyhere" localhost:443/v1/schema/write -d @write_schema.json
$ curl -X POST -H "Authorization: Bearer somerandomkeyhere" localhost:443/v1/schema/read -v
$ curl -X POST -H "Authorization: Bearer somerandomkeyhere" localhost:443/v1/relationships/write -d @write_relationships.json
$ curl -H "Authorization: Bearer somerandomkeyhere" -X POST localhost:443/v1/permissions/check -d @check.json
$ kill %1

TODO:

• Generate Gateway & OpenAPI in authzed-go
• add spicedb serve --http-addr
• Copy over authorization header to gRPC
• Copy over OpenTelemetry header to gRPC
• Serve OpenAPI schema endpoint
• Record HTTP metrics via Prometheus
• Support flags for configuring TLS
• Cleanup go.mod

[josephschorr]

This should all be skippable; either if the http server endpoint is either empty or we add a new flag

[jzelinskie]

Another idea is that we could run the dashboard, metrics, and HTTP API all on the same port.

[josephschorr]

I assume this change needs to be removed?

[jzelinskie]

yep this will remain a draft until the the following are merged:

• Add OpenAPI metadata api#9
• generate openapi and add an embed fs for it authzed-go#33

[josephschorr]

server?

[ecordell]

LGTM