Add a way to provide a certificate authority for kubectl configuration

When configuring the external api server for kubernetes in onyxia, the configuration script uses the flag ` --insecure-skip-tls-verify=true` :

```
kubectl config set-cluster api \
--server=https://api \
--insecure-skip-tls-verify=true

```

We would like to be able to provide a certificate in onyxia configuration so that this flag is not needed. For instance:

```
"k8sPublicEndpoint":
{
"URL": "https://api.kube.groupe-genes.fr",
"certificateAuthority": "LS0tLS1CRUdJTiBDRVJUS...",
"oidcConfiguration":
{
"issuerURI": "https://auth.groupe-genes.fr/realms/genes",
"clientID": "onyxia",
},
},
```

would gives :
```
kubectl config set-cluster api \
--server=https://api \
--certificate-authority=LS0tLS1CRUdJTiBDRVJUS...
```