refactor(x/staking): migrate UnbondingType to collections

[likhita-809]

Description

Closes: #16388

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[facundomedica]

This doesn't look backwards compatible, we should store a uint64 instead of int64.

-- KEY: 390000000000000063 Value: 0000000000000001 
++ KEY: 390000000000000063 Value: 8000000000000001 

I suggest you check with this to find if your changes are compatible:

func (s *KeeperTestSuite) TestColls() {
	s.SetupTest()

	err := sdktestutil.DiffCollectionsMigration(
		s.ctx,
		s.key,
		100,
		func(i int64) {
			err := s.stakingKeeper.SetUnbondingType(
				s.ctx,
				uint64(i),
				types.UnbondingType_UnbondingDelegation,
			)
			s.Require().NoError(err)
		},
		"{insert original hash here}",
	)

	s.Require().NoError(err)
}

[facundomedica]

LGTM, maybe not for this PR but it would be good to explore the possibility to make UnbondingType a uint64 instead of int, so we don't have to type cast and we wouldn't need GetUnbondingType and SetUnbondingType.

[julienrbrt]

utACK, one nit.

[julienrbrt]

Could you fix the conflicts?