feat: configurable tls port

[robertgraeff]

What this PR does / why we need it:

With this pull request, the TLS port on which the gateways are listening is configurable. The configuration of the TLS port is optional. The default port is 9443.

apiVersion: gateway.openmcp.cloud/v1alpha1
kind: GatewayServiceConfig
metadata:
  name: gateway
spec:
  clusters:
    ...
  envoyGateway:
    ...
  dns:
    ...
  gateway:
    tlsPort: <CONFIGURED TLS PORT>  # optional, default 9443

Effect on the gateway:

apiVersion: gateway.networking.k8s.io/v1
kind: Gateway
metadata:
  annotations:
    ...
    gateway.openmcp.cloud/tls-port: <CONFIGURED TLS PORT>
  ...
spec:
  ...
  listeners:
  - name: tls
    port: <CONFIGURED TLS PORT>
    protocol: TLS
    ...

Which issue(s) this PR fixes:

Fixes #15

Special notes for your reviewer:

Release note:

-  The TLS port on which the gateways are listening is configurable.

[ValentinGerlach]

Nitpicking: Can we change this to int? Then we don't need the int(...) conversion in pkg/envoy/config.go.

[ValentinGerlach]

Just noticed that the Gateway API is using int32. Then we can also leave it like that :)