[Feat.] Enterprise VPN Connection Monitoring and EVPN Tags

[anton-sidelnikov]

What this PR does / why we need it

Implements https://docs.otc.t-systems.com/virtual-private-network/api-ref/api_reference_enterprise_edition_vpn/apis_of_enterprise_edition_vpn/vpn_connection_monitoring/index.html
https://docs.otc.t-systems.com/virtual-private-network/api-ref/api_reference_enterprise_edition_vpn/apis_of_enterprise_edition_vpn/tags/index.html
https://docs.otc.t-systems.com/virtual-private-network/api-ref/api_reference_enterprise_edition_vpn/apis_of_enterprise_edition_vpn/quota/index.html

Which issue this PR fixes

Part of #724

Special notes for your reviewer

=== RUN   TestConnectionLifecycle
    connection_test.go:189: Attempting to CREATE EIP 1
    connection_test.go:197: Attempting to CREATE EIP 2
    connection_test.go:221: Attempting to CREATE Enterprise VPN Gateway: acc_evpn_gateway_9sE3F
    connection_test.go:174: Attempting to CREATE Enterprise VPN Customer Gateway: acc_evpn_cgw_f9PAY
    connection_test.go:70: Attempting to CREATE Enterprise VPN Connection: acc_evpn_connection_1cd
    connection_test.go:81: Attempting to GET Enterprise VPN Connection: e353293f-c6c3-4892-a1ef-91f85b0a1f85
    connection_test.go:88: Attempting to UPDATE Enterprise VPN Connection: e353293f-c6c3-4892-a1ef-91f85b0a1f85
    connection_test.go:105: Attempting to LIST Enterprise VPN Connections
    connection_test.go:112: Attempting to CREATE Enterprise VPN Connection Monitoring
    connection_test.go:124: Attempting to GET Enterprise VPN Connection Monitor: 70376d91-d75c-4c13-a332-bf8645574a3b
    connection_test.go:129: Attempting to LIST Enterprise VPN Connection Monitor: 70376d91-d75c-4c13-a332-bf8645574a3b
    connection_test.go:134: Attempting to GET Enterprise VPN Quotas: 70376d91-d75c-4c13-a332-bf8645574a3b
    connection_test.go:145: Attempting to CREATE Enterprise VPN Connection Tag: [{One gopher}]
    connection_test.go:153: Attempting to LIST Enterprise VPN Connection Tags: e353293f-c6c3-4892-a1ef-91f85b0a1f85
    connection_test.go:150: Attempting to DELETE Enterprise VPN Connection Tag: [{One gopher}]
    connection_test.go:119: Attempting to DELETE Enterprise VPN Connection Monitoring: 70376d91-d75c-4c13-a332-bf8645574a3b
    connection_test.go:76: Attempting to DELETE Enterprise VPN Connection: 270fac6b-c4df-488e-8123-66782977983b
    connection_test.go:179: Attempting to DELETE Enterprise VPN Customer Gateway: 5ac989d7-22e1-42d7-8b9a-4e2c9de2b88e
    connection_test.go:227: Attempting to DELETE Enterprise VPN Gateway: 270fac6b-c4df-488e-8123-66782977983b
    connection_test.go:201: Attempting to DELETE EIP 2: 1d16bea3-b486-45ad-9c3c-d6553a0906af
    connection_test.go:193: Attempting to DELETE EIP 1: ff36b36f-a46f-439f-9cdc-26ae1820f4d3
--- PASS: TestConnectionLifecycle (336.75s)
PASS

Process finished with the exit code 0