feat: As a user, I want APISIX to support the head HTTP request, so that we can check if the APISIX is available easily

### Description

As a user, I want APISIX to support the head HTTP request, so that we can check if the APISIX is available easily.

After that, the user can send a head request like below to APISIX and check if it works.
```
$ curl --head localhost:9180

HTTP/1.1 200 OK
Date: ...
...
```