fix: simplify Metadata type

[jessicamcinchak]

changes:

• break definition out into extensible interfaces BaseMetadata and PlanxMetadata; this feels easier to scale to future contributors & requirements are less coupled to Planx expectations
• remove redundant properties like metadata.service.name (instead inferrable from data.application.type)

outstanding questions / follow ups:

• get an example of Planning Data's organisation curie - can we better type/document expected value for this?
• are createdAt / submittedAt redundant? does the end-consumer actually care about the length of time between starting and submitting the application? should this is simply be one date that indicates when this payload is being submitted?