Reuse rpcv6 BlockHeader for v7 and v8

[hudem1]

clean rpc pkgs according to issue #2437

• rpcv7's Header is the same as v6, so, i just removed it and reused v6's entirely.
• rpcv8's Header just has one more field L2GasPrice. So, I embedded rpcv6.Header into rpcv8.Header. Let me know whether it is ok to do that. I thought it could be nice as it reuses v6's Header and reduces code dup and makes it clear what new fields have been added, but when instantiating a v8 Header, we have to define an rpcv6 Header field inside, so it might be a bit strange maybe? ie:

rpcv8.BlockHeader {
   BlockHeader: rpcv6.BlockHeader { ... },
   L2GasPrice: ...,
}

And also, as you can see in the tests (in file subscriptions_test.go) that i had to fix, it changes the order of the fields in the stringified json. Could that be an issue ? I don't think so but still asking to make sure it's fine!

[codecov]

Codecov Report

Attention: Patch coverage is 96.96970% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.52%. Comparing base (6248280) to head (1b81f50).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
rpc/v8/block.go	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2448      +/-   ##
==========================================
+ Coverage   73.44%   73.52%   +0.07%     
==========================================
  Files         140      140              
  Lines       17143    17138       -5     
==========================================
+ Hits        12591    12601      +10     
+ Misses       3671     3655      -16     
- Partials      881      882       +1     

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