add flag --ca-crt-file to flux create secret git

Add flag `--ca-crt-file` to `flux create secret git` to specify the path
to CA certificate. It takes precedence over `--ca-file` and uses the
key `ca.crt` in the generated Secret.

Signed-off-by: Sanskar Jaiswal <jaiswalsanskar078@gmail.com>

cmd/flux/testdata/create_secret/git/secret-ca-crt.yaml

@@ -0,0 +1,11 @@
+---
+apiVersion: v1
+kind: Secret
+metadata:
+  name: ca-crt
+  namespace: my-namespace
+stringData:
+  ca.crt: ca-data
+  password: my-password
+  username: my-username
+