Customizable keychain item name

**Is your feature request related to a problem you're having? Please describe.**
Currently library is storing the token within the keychain using the hardcoded name "auth" that is not customizable, we would like to test different versions (debug/release) of the application at the same machine and sometimes we found the system in a broken state, where the token is not correctly identified.

**Describe the solution you'd like**
We would like a bundle configuration control over the name of the keychain's item. Ideally, adding a new key to Info. list would control which name the keychain item should have. (right now, it's always equal to "auth", and it's set via `kGTMAppAuthKeychainName` const)

**Describe alternatives you've considered**
Alternatives would be using a different library for authorization

**Additional context**