Allow usage of same tls certificate for multiple listeners in a gateway for same hostname

*Description*:
I'm trying to deploy a Gateway with two listeners with tls mode set to `Terminate`.

The idea is to use the same certificate for both listeners, because later we will have more listeners and each listener would then need it's own certificate. That doesn't feel so good.

```yaml
apiVersion: gateway.networking.k8s.io/v1
kind: Gateway
metadata:
annotations:
cert-manager.io/cluster-issuer: acme
name: my-gateway
namespace: envoy-gateway
spec:
gatewayClassName: gwc-envoy
listeners:
- allowedRoutes:
namespaces:
from: All
hostname: bla.fasel.de
name: hec
port: 443
protocol: HTTPS
tls:
certificateRefs:
- group: ""
kind: Secret
name: tls-cert
mode: Terminate
- allowedRoutes:
namespaces:
from: All
hostname: bla.fasel.de
name: syslog-tcp-tls-source-1
port: 1234
protocol: TLS
tls:
certificateRefs:
- group: ""
kind: Secret
name: tls-cert
mode: Terminate
```

Unfortunately this doesn't work:
```
No valid secrets exist: envoy-gateway/tls-cert public key algorithm must be unique, certificate domain bla.fasel.de has a conflicting algorithm [RSA].
```

I wonder why this limitation is in place. Shouldn't i be able to use the same certificate on two different listeners for the same hostname?

I'm using the envoy-helm-image in version 1.8-rc1