Update remote signer for 0.11

[mcdee]

This re-implements the changes for the remote keymanager to work with the 0.11 spec as per the current testnet.

Original PR was #5260 . Notes from that PR (which was for the 0.10 spec):

A few updates for the remote keymanager after running it in test for a while:

• increase the maximum message size received from the remote server to allow for larger number of keys
• do not panic on bad response from remote server
• check response code when asking for accounts and error on error situations (requires update to eth2-signer-api dependency)
• when receiving list of accounts to validate with from the remote server check them against user-supplied path specifier to ensure we aren't validating accounts we shouldn't

In addition this provides the proposer index as part of beacon proposal signing request, as per the change in 0.11.

Part of #5423

[codecov]

Codecov Report

Merging #5602 into master will not change coverage.
The diff coverage is n/a.

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##           master    #5602   +/-   ##
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  Coverage   53.48%   53.48%           
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  Files         309      309           
  Lines       25575    25575           
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  Hits        13680    13680           
  Misses       9866     9866           
  Partials     2029     2029