Don't export coerceKeyRole, export RewardAccount

[smelc]

Changelog

- description: |
    Don't export the ledger's `coerceKeyRole` function anymore, export RewardAccount
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

• Required for Augment of query spo-stake-distribution to include the DRep delegation choices of the Pool's rewards accounts cardano-cli#990
• This is the datatype RewardAccount that is being exported
• The export of coerceKeyRole is no longer necessary for solving [FEATURE] - Update cardano-cli conway query spo-stake-distribution cardano-cli#911, which is good; because @Jimbo4350 had concerned about it (on Slack). I must say I readied Export HasKeyRole's coerceKeyRole function #699 too fast.

Not marking this one as breaking, because it actually reverts #699. Since there was no release since 699, overall it's not going to be a breaking change.

How to trust this PR

The new export only concerns a data structure, nothing controversial

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• Self-reviewed the diff

[palas]

Looks good, but it does seem breaking. It would need to be noted in the changelog

[smelc]

@Jimbo4350> confirmed with lehins that the only valid use of coercing key roles is for the Witness role. But even then, Alexei strongly supports not exporting it:

That is the whole point of the key roles. It should not be needed and usually when people try to do it, that means that they are doing something wrong.

In other words by exposing such casting we would promote people making mistakes