feat: modified getMessage to allow setting baseUrl

[ukyoda]

I appreciate the availability of this library. Thanks to all the maintainers.

I have been using the getResponse function lately. It is a very useful API, but I thought it would be better if I could also use handlers created with relative paths.

So I would like to add a baseUrl argument to getResponses. I think this can be adapted without causing side-effects, since it just allows existing implementations to be used.

Usage:

const response = await getResponse(
  [http.get('/user', () => Response.json({ name: 'John' }))],
  new Request('http://localhost/user'),
  'http://localhost',
)

If this is not a problem, I would love to have it merged. Please review it.

I am not good at English and I don't have much experience in Contribute to OSS, so I am sorry if I am rude.