Add model to HTTP library for "problem details"

[mikekistler]

RFC 7807 Problem Details for HTTP APIs defines a standard error response format for HTTP APIs. The TypeSpec HTTP library should include a model that describes this format.

[markcowl]

est: 5

[mikekistler]

Here's a proposal for the model for Problem Details:

/**
 * Base error response model as defined in RFC 7807 Problem Details for HTTP APIs.
 */
model ProblemDetails {
  @header("content-type") contentType: "application/problem+json";

  /**
   * A URI reference [RFC3986] that identifies the problem type.
   */
  type?: string;

  /**
   * A short, human-readable summary of the problem type.
   */
  title?: string;

  /**
   * The HTTP status code for this occurrence of the problem.
   */
  status?: integer;

  /**
   * A human-readable explanation specific to this occurrence of the problem.
   */
  detail?: string;

  /**
   * A URI reference that identifies the specific occurrence of the problem.
   */
  instance?: string;
}