added support for zod object for response headers

[AGalabov]

Resolves #116.

Side note: right now there are 3 approaches to generate response headers:

1. Directly specifying a HeadersObject through the response

responses: {
  204: {
    description: 'Success',
    headers: { 'Set-Cookie': { schema: { type: 'string' } } },
  },
},

2. Using custom components:

const apiKeyHeader = registry.registerComponent('headers', 'api-key', {
      example: '1234',
      required: true,
    });

// ...
responses: {
  200: { description: 'Sample response', headers: { 'x-api-key': apiKeyHeader.ref }  }
};

3. Using ZodObject:

responses: {
  204: {
    description: 'Success',
    headers: z.object({
      'Set-Cookie': z.string().openapi({ example: 'token=test' }),
    }),
  },
},

However I do feel that all are valid because:

1. Obviouslly we should allow plain documentation to be written as everywhere else in the codde
2. The custom schemas are a mechanism that can be utilized for response headers but are not necessarily made for that
3. Adding support for ZodObject seems logical since this is something that could be validated and it is a convenience to support.

[georgyangelov]

Nice