opencensus-ext-google-cloud-clientlibs: add support for google-cloud-core >= 2.0.0

[dariemp]

Before opening a feature request against this repo, consider whether the feature should/could be implemented in the other OpenCensus client libraries. If so, please open an issue on opencensus-specs first.

Is your feature request related to a problem?
The Opencensus extension for Google Cloud client libraries (opencensus-ext-google-cloud-clientlibs) is not compatible with the most recent versions of the Google Cloud API:

• opencensus-ext-google-cloud-clientlibs install requirements: 'google-cloud-core ~= 1.7; python_version == "2.7" or python_version >= "3.6"')

But the latest version of google-cloud-core is 2.3.2. This incompatibility makes it impossible to upgrade Google Cloud libraries in projects using this Opencensus extension.

Describe the solution you'd like.
Please, add support for the most recent Google Cloud libraries (google-cloud-core >= 2.0.0)

Describe alternatives you've considered.
I've considered stopping using Opencensus

Additional context.
Our project requires an upgrade to the latest Google Cloud client libraries but this Opencensus extension is blocking us.