[pipedv1] Support plugin metadata in application data model and protobuf schema

Parent Epic / Context

This issue is a sub-task derived from Epic #5252 (Make PipeCD control plane support plugin-arched piped) and Epic #5259.

What would you like to be added?

Update the application data model and Protobuf definitions in pkg/model to accommodate pipedv1 plugin metadata (plugins and deployTargets) while preserving backward-compatibility for legacy pipedv0 fields (platform_provider).

Why is this needed?

To allow the PipeCD Control Plane to handle both pipedv0 and pipedv1 agents simultaneously during the migration phase, the underlying application data structures must support plugin metadata without requiring legacy platform provider fields.

Proposed Solution & Implementation Plan

1. Protobuf & Schema Updates: Update application .proto files in pkg/model to include plugins and deployTargets.
2. Validation Logic: Update validation rules in pkg/model/application.go so that pipedv1 plugin-based application configurations pass validation without requiring platform_provider.
3. Unit Tests: Add unit tests under pkg/model verifying marshaling, unmarshaling, and validation for both pipedv0 and pipedv1 application entities.

Acceptance Criteria

• Protobuf definitions generate cleanly via project codegen tools.
• pipedv1 application configs pass validation without requiring platform_provider.
• Existing pipedv0 unit tests pass without regression.

Start with the application .proto files and application.go under pkg/model, then identify the project's code generation command. Add support for pipedv1 plugins and deployTargets while preserving pipedv0 platform_provider compatibility. Add pkg/model tests for marshaling, unmarshaling, and validation, and confirm code generation and existing pipedv0 tests pass.