feat: types.HeaderHooks JSON round-trip support

[ARR4N]

Why this should be merged

JSON equivalent of #89.

How this works

The check for registered extras, previously used in {En,De}codeRLP() methods is abstracted into a Header.hooks() HeaderHooks method that either returns (a) an instance of the registered type or (b) a NOOPHeaderHooks if no registration was performed. This is then used for all hooks, new (JSON) and old (RLP).

How this was tested

Extension of existing unit tests.

[qdm12]

nit no need to assign to r:

Suggested change

	if r := registeredExtras; r.Registered() {
	return r.Get().hooks.hooksFromHeader(h)
	}
	if registeredExtras.Registered() {
	return registeredExtras.Get().hooks.hooksFromHeader(h)
	}

[ARR4N]

I did it to avoid unnecessary verbosity and code stuttering. One of the core guiding principles that I follow (and recommend) is for code to get out of the way of the human seeing the meaning being communicated to the computer. Here there's a trade-off between distraction (a 5-syllable variable being unnecessarily repeated, with 3 of them repeated again in the method name) and a meat-RAM allocation (remembering what r is). I chose the latter because it only needs to be remembered for one line.

[qdm12]

I was actually saying this for readability more than for memory usage 😄
Assigning to r made me think initially we were copying the variable because the method calls were perhaps modifying it. I feel it could hide something eventually if you see what I mean?

[qdm12]

🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔

[ARR4N]

Well?

[qdm12]

Any hint on an encoding?
Is this your 256 bits private key? 😄