Improve Type support

[chimurai]

Provide better Type support. Not only for express, but also fastify, next.js, etc...

Related issues

• https://github.com/chimurai/http-proxy-middleware/pull/478
• https://github.com/chimurai/http-proxy-middleware/pull/683
• https://github.com/chimurai/http-proxy-middleware/issues/692
• https://github.com/chimurai/http-proxy-middleware/issues/650
• https://github.com/chimurai/http-proxy-middleware/pull/603
• https://github.com/chimurai/http-proxy-middleware/issues/632
• https://github.com/chimurai/http-proxy-middleware/pull/687
• https://github.com/chimurai/http-proxy-middleware/pull/700
• https://github.com/chimurai/http-proxy-middleware/issues/708
• https://github.com/chimurai/http-proxy-middleware/issues/713
• https://github.com/chimurai/http-proxy-middleware/issues/719