Feature/external native schema files

[bascunansalvador]

The previous implementation required native Avro and native Protobuf schemas to be written inline inside the AsyncAPI document. That worked for small examples, but it was not ideal for real projects where .avsc and .proto files are usually maintained as standalone schema assets.

This was also inconsistent with how teams normally work with native schema formats. Avro and Protobuf schemas often have their own tooling, validation, build steps, and ownership, so forcing the schema text into the AsyncAPI YAML made the contract harder to maintain.

For the proposed solution, native Avro and native Protobuf schemas can now be referenced from the AsyncAPI document through schema.$ref.

For example, native Avro schemas can now be written like this:

components:
  schemas:
    UserCreated:
      schemaFormat: application/vnd.apache.avro+json;version=1.9.0
      schema:
        $ref: schemas/UserCreated.avsc

Native Protobuf schemas can be written in the same style:

components:
  schemas:
    UserCreated:
      schemaFormat: application/vnd.google.protobuf;version=3
      schema:
        $ref: schemas/user-created.proto

The external schema path is resolved relative to the AsyncAPI file that owns the reference. This reuses the existing external-reference path handling so native schema assets follow the same project-local reference behavior as other external files.

The parser now reads external native schema assets as plain text instead of routing them through the YAML or JSON document reader. This is important because .avsc and .proto files are native schema assets, not AsyncAPI documents.

A domain-specific parse error was also added for missing or unreadable native schema files. This means users get a clear generator error when a referenced .avsc or .proto file cannot be read, instead of a low-level file or parsing failure.

The generator output contract was extended so external Avro files generate .avsc output and Java SpecificRecord sources, and external Protobuf files generate .proto output.

Spring Kafka generation was also covered end to end. Java and Kotlin Spring Kafka clients now resolve payload types from external native Avro and Protobuf schema files in both full and simple client modes.

The README was updated to document both inline native schema usage and external native schema file references.

The test coverage includes parser tests for external native Avro and Protobuf schema assets, a missing-file parser diagnostic test, generator output contract tests for external .avsc and .proto assets, and Spring Kafka runtime generation tests for Java and Kotlin full/simple clients.