fix: fix resource path args for paths with =**

[busunkim96]

Some resources have =** in the segment. . A segment with =** is not matched by the current regex:

Python 3.9.5 (default, Jul 27 2021, 22:06:04) 
[GCC 10.2.1 20210110] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import re
>>> pattern = re.compile(r"\{([a-zA-Z0-9_-]+)\}")
>>> pattern.search("projects/{project}/metricDescriptors/{metric_descriptor=**}").groups()
('project',)

This pattern shows up in the some real APIs (monitoring protos, google/cloud/iap/v1, google/cloud/iap/v1beta1, google/cloud/recommendationengine/v1beta1, google/devtools/remoteworkers/v1test2, google/home/graph/v1, google/iam/v1). ** is mentioned in passing in Resource Names. I was not able to find an explanation of what wildcards are considered valid in https://google.aip.dev/122 or https://google.aip.dev/client-libraries/4231.

Monitoring Proto Example:

option (google.api.resource_definition) = {
  type: "monitoring.googleapis.com/MetricDescriptor"
  pattern: "projects/{project}/metricDescriptors/{metric_descriptor=**}"
  pattern: "organizations/{organization}/metricDescriptors/{metric_descriptor=**}"
  pattern: "folders/{folder}/metricDescriptors/{metric_descriptor=**}"
  pattern: "*"
  history: ORIGINALLY_SINGLE_PATTERN
};

[busunkim96]

This regex seemed to be serving the same purpose as the one in wrappers.py