feat: allow @ExampleObject values to be loaded from a file (#5180)

[kuntal1461]

Summary

• Add externalFile() field to @ExampleObject annotation to load example payloads from classpath or filesystem files
• Modify AnnotationsUtils.resolveExample() to read the file content and process it identically to the inline value field (JSON-parsed, falling back to raw string)
• Supports classpath: and file: prefixes; bare paths default to classpath
• When both externalFile and value are set, externalFile takes precedence; gracefully falls back to value if the file is not found

Motivation

Closes #5180. For APIs with large payloads, defining examples inline in @ExampleObject(value = "...") is unmanageable — it requires defining enormous string constants in Java classes. This feature lets users store example payloads in separate files:

@ExampleObject(
    name = "Create User",
    externalFile = "classpath:openapi/examples/create-user-request.json",
    summary = "Full user creation example"
)

Test plan

• Unit tests added to AnnotationsUtilsTest: file-found (JSON parsed as JsonNode), externalFile precedence over value, missing-file fallback to value, classpath: prefix handling
• Integration test added to ExamplesTest: ExternalFileRequestBodyExample loads JSON from examples/external/user-request.json and verifies OpenAPI YAML output
• All 917 existing tests pass (swagger-core: 691, swagger-jaxrs2: 226)
• Java 8 compatible (uses ByteArrayOutputStream + byte-loop instead of InputStream.readAllBytes())

[Mattias-Sehlstedt]

Note: I am not affiliated with this repo. These are my own opinions.

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I believe file handling is outside of what AnnotationUtils responsibilities, and that it would make sense to extract this to some separate file-handling util class. This so that the AnnotationUtils only know "I should fetch this from a file", but it does not have to consider itself in any way of how files are actually fetched and parsed.