Override _alterDelegatee

[funkyenough]

• Override _alterDelegatee to set validator context for earning power calculator context
• Add tests

Resolves #36

[github-actions]

Coverage after merging fix/alter-delegatee-validator-context into feat/registry-integration will be

85.86%

Coverage Report

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
src
ZkStaker.sol	86.31%	71.43%	94.44%	85.86%	484, 490, 663, 668–669, 671, 673, 682, 687, 687, 689, 691–692, 694, 696, 699, 699–700, 703, 703–704, 708–709, 709–710, 713, 718, 720, 723, 726–727