Add support for ~ separated resource names

[nwbirnie]

In ads API, we use resource names which contain ~ to delimit multiple IDs in a path segment. For example:

message AdGroupAd {
  option (google.api.resource) = {
    type: "googleads.googleapis.com/AdGroupAd"
    pattern: "customers/{customer_id}/adGroupAds/{ad_group_id}~{ad_id}"
  };

The current microgenerator implementation doesn't appear to support this, and instead just generates a single param in its place.

Link to protos: http://shortn/_zIPrXkqkoU

[miraleung]

Fixed in #450