Add capability to specify proxy for Artifactory repository replications

[Guent4]

Is your feature request related to a problem? Please describe.
I have a use case where I am trying to configure Artifactory replications through Terraform and need to be able to set a proxy for the replication.

Describe the solution you'd like to see
Add support for setting the proxy when creating a new replication. After support is added in this repo, changes will also be needed for https://github.com/jfrog/terraform-provider-artifactory.

Describe alternatives you've considered
N/A

Additional context

• Add Proxy and ChartsBaseUrl to Replication and Virtual Repo models atlassian/go-artifactory#33 tried adding support for it when terraform-provider-artifactory was using that repo for the client. We were using that until we upgraded to Artifactory 7.x where the JSON API for GET and POST replications was slightly changed.

[Guent4]

PR here: #524