Add implementation for GW visibility control feature

[npamudika]

This PR adds implementation for the GW visibility control feature.

To set the visibility, use the following config in the deployment.toml. You need to provide visibility roles as an array as below.

[[apim.gateway.environment]]
name = "gateway-1"
type = "hybrid"
visibility = ["role-1","role-2","role-3"]
gateway_type = "Regular"
provider = "wso2"
display_in_api_console = true
description = "This is a hybrid gateway that handles both production and sandbox token traffic."
show_as_token_endpoint_url = true
service_url = "[https://localhost:${mgt.transport.https.port}/services/](https://localhost:$%7Bmgt.transport.https.port%7D/services/)"
username= "${admin.username}"
password= "${admin.password}"
ws_endpoint = "ws://localhost:9099"
wss_endpoint = "wss://localhost:8099"
http_endpoint = "[http://localhost:${http.nio.port}](http://localhost:$%7Bhttp.nio.port%7D/)"
https_endpoint = "[https://localhost:${https.nio.port}](https://localhost:$%7Bhttps.nio.port%7D/)"
websub_event_receiver_http_endpoint = "http://localhost:9021/"
websub_event_receiver_https_endpoint = "https://localhost:8021/"

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Naduni Pamudika seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[npamudika]

This PR is related to wso2/api-manager#3378

[tharindu1st]

This doesn't need to be there lets retrieve from environments

[npamudika]

Fixed