The validator client was only able to connect to beacon nodes exposing
the exact same set of spec constants that are locally known via their
config/spec REST API. However, that set of spec constants is dynamic.
As the validator client only requires a subset of relevant constants,
this may lead to compatible specs being rejected. This patch widens the
allowed specs by only verifying that the required set of constants are
present in the spec response, ignoring any spec constants that are not
locally known, and ignoring missing spec constants that are locally
known but not included by the remote beacon node when not relevant for
operation of the validator client.
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