Added a new snippet for ExpressRoute Gateways, Microsoft.Network/expressRouteGateways.

Contributing a snippet

• I have a snippet that is either a single, generic resource or multi resource that uses parent-child syntax

• I have checked that there is not an equivalent snippet already submitted

• I have used camelCasing unless I have a justification to use another casing style

• I have placeholders values that correspond to their property names (e.g. dnsPrefix: 'dnsPrefix'), unless it's a property that MUST be changed or parameterized in order to deploy. In that case, I use 'REQUIRED' e.g. keyData

• I have my symbolic name as the first tab stop ($1) in the snippet. e.g. res-aks-cluster.bicep

• I have a resource name property equal to "name"

  e.g.

  resource aksCluster 'Microsoft.ContainerService/managedClusters@2021-03-01' = {
    name: 'name'