Add query for the max major protocol version

[amesgen]

IntersectMBO/cardano-cli#773 motivates a query to obtain the maximum major protocol version supported by one's node. The goal of this ticket is to add such a query.

It is part of the (partial) ConsensusConfig, e.g. for Praos:

ouroboros-consensus/ouroboros-consensus-protocol/src/ouroboros-consensus-protocol/Ouroboros/Consensus/Protocol/Praos.hs

Lines 199 to 201 in 61c990c

	-- | All blocks invalid after this protocol version, see
	-- 'Globals.maxMajorPV'.
	praosMaxMajorPV :: !MaxMajorProtVer,

There are at least two ways to accomplish this:

• Add a Shelley BlockQuery constructor to get just the MaxMajorProtVer.

  Most direct and least amount of work, but ad-hoc.

• Add a GetConsensusConfig query, similar to GetLedgerConfig as added in Add new GetLedgerConfig query #1394.

  Potentially more principled and avoiding additional ad-hoc queries for part of the ConsensusConfig, but more work, and maybe YAGNI.

  EDIT: Probably shouldn't go with this option: Downgrade GetLedgerConfig to a debug query #1478