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