request help: How to get the cert and cacert from the plugin request

### Issue description
It seems we can't get the certs, `curl --cert file1.crt --cacert file2.crt`, from the [request](https://pkg.go.dev/github.com/apache/apisix-go-plugin-runner@v0.5.0/pkg/http#Request) of the default plugin.

In this case, we can only load the cert content in the header, which is not convenient. So is it possible to extend it for obtaining certificates based on the file path?

### Environment

* APISIX Go Plugin Runner's version:
* APISIX version:
* Go version:
* OS (cmd: `uname -a`):