Support Push based transport mode

[Fannon]

Push Transport Mode for ORD

This PR adds Push Transport as a new transport mode for ORD, enabling providers to push ORD documents directly to aggregators via HTTP POST requests.

Summary

Push transport eliminates the need for providers to implement and host an ORD Provider API. Instead, providers can:

• Push ORD documents during CI/CD pipelines (design-time or deploy-time)
• Include resource definitions inline via the new definitions property
• Receive immediate validation feedback from the aggregator

Changess

• Added definitions property at document level for inline resource definitions
  • Dictionary structure: URL path as key, raw content as string value
  • Marked as x-introduced-in-version: 1.15.0 with beta status
• Added embedded access strategy type for resource definitions
• New "Push Transport" section with:
  • Use cases and motivation
  • Pros/cons comparison with pull transport
  • Implementation details for ORD Push Document format
  • Push API contract specification
  • Provider authorization requirements
  • Sequence diagram showing CI/CD integration flow

Design Decisions

1. definitions as dictionary - URL path as key enables direct correlation with resourceDefinitions[].url
2. String values only - Content is an opaque text blob, preserving whitespace and working uniformly across formats (JSON, YAML, XML, etc.)
3. embedded access strategy - Explicitly signals that content is inline, not to be fetched
4. Aggregator configuration deferred - Schema for aggregator .well-known endpoint will be addressed in a separate PR

Related / Follow Up

• Define Aggregator configuration schema, incl. announcing push API and its access strategy, validation endpoint