Extract base classes to reduce request and search DTO boilerplate.

[mikism]

Pull duplicated fields and request-building logic into shared abstract
base classes, leaning on Lombok's @SuperBuilder/@Getter/@EqualsAndHashCode
(callSuper = true) in place of the per-class @Value/@Builder/@DaTa setup.

Search DTOs:

• Add an abstract BaseSearch holding the fields common to ArrivalSearch
  and DepartureSearch in the postcodes, routes, timefilter, timefilterfast,
  timemap and zones packages; the concrete classes now only declare their
  distinguishing fields (e.g. arrivalTime / departureTime).
• Merge the identical timemapfast ManyToOne and OneToMany into a single
  Search class and reference it from ArrivalSearches.

Request DTOs:

• Add BaseTravelTimePostRequest centralising POST request creation behind
  abstract endpoint() and acceptType() hooks, and move createPostRequest
  out of TravelTimeRequest into it.
• Add BaseTimeMapRequest / BaseTimeMapFastRequest sharing the departure,
  arrival, intersections and unions fields plus the endpoint across the
  TimeMap(Fast) JSON, WKT, KML, GeoJSON and boxes variants.
• TravelTimeRequest gains @SuperBuilder/@NoArgsConstructor and exposes the
  JSON media type as protected for the subclasses.

Tests & fixtures:

• Update request unit tests and the expected JSON fixtures to match the new
  field serialisation order (inherited base fields are emitted before the
  subclass-specific ones).