Complete refactor of endpoint detection logic

[danopia]

• Seperate URL building out into a clean, new, tested module
• Allow for swapping out the URL-building logic entirely
• Better configurability for dual-stack such as opt-out
• Allow forcing regional endpoints for local STS
• Continue supporting basic fixedEndpoint usage

A better fix for #3 / #7

Examples

// just defaults (dualstack)
const s3 = new ApiFactory().makeNew(S3);

// disabling global endpoints for better latency
const stsRegional = new ApiFactory({
  endpointResolver: new AwsEndpointResolver({
    forceRegional: true,
  }),
}).makeNew(STS);

// non-AWS S3 backend that still has regions and host based routing
const linodeObjects = new ApiFactory({
  endpointResolver: new S3CompatibleEndpointResolver('linodeobjects.com'),
}).makeNew(S3);