Check conten-type and return 415 if not supported by route

[nflaig]

Description

Noticed that Lighthouse does not check the content type of the request body and just tries to deserialize it, causing strange errors.

E.g. Error on Lodestar when trying to call getAttesterDuties with SSZ request body

May-28 11:04:57.066[]                [31merror[39m: Failed to download attester duties epoch=0 - getAttesterDuties failed with status 400: BAD_REQUEST: body deserialize error: Error("expected value", line: 1, column: 1)
Error: getAttesterDuties failed with status 400: BAD_REQUEST: body deserialize error: Error("expected value", line: 1, column: 1)

As per spec, this route does not require to support SSZ request bodies and by default Lodestar will be using JSON but it would still be nice if Lighthouse could return a 415 error as this allows Lodestar to retry the request with JSON and cache for this route that SSZ is not supported, only sending JSON in subsequent requests.

Clients returning a proper status code would also allow to gradually support SSZ for more routes which based on data we gathered https://github.com/ChainSafe/lodestar/pull/6749#issuecomment-2128847283 would be quite benefitial.

Version

v5.1.3

Present Behaviour

Lighthouse tries to parse body without checking first if content type is supported

Expected Behaviour

Check Content-Type header and return 415 error if content type is not supported by route.

Steps to reproduce

I've been using Kurtosis to run the tests (with custom lodestar image)

participants:
  - el_type: geth
    el_image: ethereum/client-go:stable
    cl_type: lodestar
    cl_image: nflaig/lodestar:ssz-api
    vc_type: lighthouse
    vc_image: sigp/lighthouse:latest
    count: 1
  - el_type: geth
    el_image: ethereum/client-go:stable
    cl_type: lighthouse
    cl_image: sigp/lighthouse:latest
    vc_type: lodestar
    vc_image: nflaig/lodestar:ssz-api
    count: 1