Custom Certificate Authority for ACR Task Credentials

**What is the problem you're trying to solve**
As part of the Private Agent Pool option, ACR Tasks now can run in private network for large enterprise companies and run builds from/to private registries available in the private network.
However, for most of these registries, the certificate authority is an internal PKI, meaning that ACR Tasks as of right now cannot connect to it.

**Describe the solution you'd like**
a way to configure certificate authority bundle on the ACR Task credentials, like this:
```
{
"type": "Microsoft.ContainerRegistry/registries/tasks",
"apiVersion": "2019-06-01-preview",
"name": "string",
"location": "string",
# omited for brevity
"properties": {
"credentials": {
"customRegistries": {
"private-registry.private-dns.net" : {
"certificateAuthority": "b64 encoded CA Certificate bundle", # <------------- This line here
"username": "foo",
}
#omited for brevity
}
```

**Additional context**
Add any other context or screenshots about the feature request here.