Respecting JSONEncoder's keyEncodingStrategy

Issue:
Found an issue where when I use theOpenAPIEncodedSchemaType.openAPISchema(using encoder: JSONEncoder) to create a JSONSchema with passing an encoder that has keyEncodingStrategy set to convertToSnakeCase, the JSONSchema's keys wasn't converting the keys to use convertToSnakeCase and returning as what the variable name is set to.

How to reproduce:

1. Create a JSONEncoder with the following:

let encoder = JSONEncoder()
encoder.keyEncodingStrategy = .convertToSnakeCase

2. Create a sample model

struct SampleObject: Codable, Sampleable, OpenAPIEncodedSchemaType {
    let sampleKey: String
    let sampleValue: String
}

3. Initialize the model with any values and use the SampleObject's openAPISchema(_:) to generate the schema.
4. Verify the keys of the model as sampleKey and sampleValue instead of sample_key and sample_value.

Expected:
The expected result should be having the keys as sample_key and sample_value.

Start at OpenAPIEncodedSchemaType.openAPISchema(using encoder: JSONEncoder) and reproduce the issue with JSONEncoder.keyEncodingStrategy set to .convertToSnakeCase and the SampleObject shown. Done means the generated JSONSchema uses sample_key and sample_value rather than the Swift property names.