Support ca.crt in TLS secret for Istio mTLS gateways
# Feature Request
## Why
Currently you can configure MUTUAL mode for gateways, but there isn't a way to add the `ca.crt` to the TLS secret.
Reference: https://repo1.dso.mil/platform-one/big-bang/bigbang/-/blob/master/chart/templates/istio/controlplane/secret-tls.yaml
## Proposed Solution
Bigbang Override could potentially look similar to the following (adding the `ca`):
```
gateways:
mtls-ingressgateway:
app: mtls-ingressgateway
hosts:
- '*.example.com'
ingressGateway: mtls-ingressgateway
tls:
mode: MUTUAL
cert: |
REDACTED
key: |
REDACTED
ca: |
REDACTED
```
issue