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/"

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[npamudika]

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