[Feature] Provide high-level methods for identifying cloud run and cloud functions

In the design of this library, class methods of Google::Cloud::Env are generally used for high-level semantic queries, such as "am I running on Cloud Run?"

We should provide some of these for the modern hosting environments such as Cloud Run and Cloud Functions. I believe some criteria that can be used include:

* GCF first gen: `GAE_RUNTIME`, `K_SERVICE`, `K_REVISION` set, and functions framework present
* GCF second gen: `GAE_RUNTIME`, `K_SERVICE`, `K_REVISION`, `K_CONFIGURATION` set, and functions framework present
* GCR first gen: `K_SERVICE`, `K_REVISION`, `K_CONFIGURATION` set, smbios info absent
* GCR second gen: `K_SERVICE`, `K_REVISION`, `K_CONFIGURATION` set, smbios info present