Add SSZ support to builder api

[nflaig]

This is a proposal to add SSZ support to builder API in a backward compatible way without requiring to bump the apis to v2.

It outlines what is expected from the client and how a well-behaved server should handle different content types, headers, and use appropriate error codes to allow the client to handle the error.

Notable changes are

• submitBlindedBlock allows to SSZ encoding for both request and response body
• getHeader allows SSZ encoding of response body
• Define error codes for unsupported media types (406, 415), those are not mandatory but it would be good if servers adopt these over time to allow clients to implement specific error handling
• Adds Eth-Consensus-Version to responses, only required if response body is SSZ encoded but preferably should be added to all responses eventually
• Add notes on how a well-behaved client and server implementation should work

Since we can't know if a builder implemented the changes as per spec, the client can determine if SSZ is supported by requesting an SSZ encoded response in the getHeader request. Preferably this should be done setting a Accept header with multiple entries and quality values like Accept: application/octet-stream;q=1.0,application/json;q=0.9 to give the builder the option to send a JSON response.

Related to #53 and #54

[metachris]

In support of this change. It's nicely backwards-compatible, doesn't add significant risk, decreases latency and could simplify CL flows.

[ltitanb]

Full support, it's a great improvement over the existing API and should be particularly useful for larger blocks. Maybe worth adding an example payload for SSZ as well?

[nflaig]

Maybe worth adding an example payload for SSZ as well?

I gave this a try but it has it's limitations since you can't display binary data on the explorer. The solution seems to be to use external examples so we could add ssz to existing examples and reference the files in the ssz examples.

However, there seems to be an issue swagger-api/swagger-ui#5433, the examples won't be displayed on the explorer, it would be great if it would show an option to download the example file but it only shows no example available.

There might be a fix for this at some point like swagger-api/swagger-js#2013. For now could do something like here nflaig/builder-specs@ssz-support...nflaig:builder-specs:ssz-examples with the caveat that it won't be available on the explorer.