Milestone tracking

[shaaibu7]

• Add struct to manage milestone details.
• Add mapping to track every milestone in an escrow contract.
• Implement logic to add milestone for an escrow.
• Implement functionality to request payment after successful completion of milestone and marking that milestone as paid.
• Implement logic to add a milestone immediately after deploying an escrow contract in the factory contract to satisfy the condition of every escrow contract having an initial milestone.

[manoahLinks]

this is looking good, in the milestone struct make the approve field to need approval of both parties