feat: add --json / --output-format=json flag for structured error output in CI pipelines

Look at the existing CLI output formatters in the codebase, likely in a module like openapi_spec_validator/cli.py or openapi_spec_validator/formatters.py. Study how spectral and vacuum implement JSON output. The first step is to add a --format json argument to the CLI parser and create a new formatter class that outputs structured error objects with the specified fields. Test by running the validator on a sample spec and verifying the JSON output is valid and contains the expected keys.

The CLI currently outputs human-readable error messages, which works well for interactive use but is harder to parse in CI/CD pipelines or editor integrations.

Issue #160 ("No specific error information available", open since 2022) highlights how vague error messages make debugging difficult. A structured JSON output mode would solve this by emitting parseable error objects with:

• path: the location in the spec where the error occurred
• message: human-readable description
• rule: the specification rule that was violated (e.g. openapi.3.1.schema-validation)
• severity: error / warning / info
• suggestion: (optional) guidance on how to fix

This could be activated via --format json or controlled through the Python API by passing a structured formatter. Existing validators like spectral (--format json) and vacuum already support this pattern.

The openapi-core ecosystem would also benefit — consumers like prism or openapi-core could consume structured validation results without string-parsing output.