Annotate Request Body messages as Required

[bshaffer]

When a request message appears in a discovery doc, for example:

message InsertForwardingRuleRequest {
  "insert": {
      //...
      "request": {
          "@ref": "ForwardingRule"
      }
      //... 
  }
  //...
}

This generates a request message with that resource on it:

message InsertForwardingRuleRequest {
  ForwardingRule forwarding_rule_resource = 32776239;
  //...
}

However, the required annotation is missing in the proto. As a result, the call in GAPIC PHP looks like the following:

$optionalParams = ['forwardingRule' => $forwardingRule];
$forwardingRulesClient->insert('project', 'region', $optionalParams);

When it would be better as this:

$optionalParams = [];
$forwardingRulesClient->insert('project', 'region', $forwardingRules, $optionalParams);

We should verify with the API teams that any message in a request body is required.

[bshaffer]

Fixed in #21