Add an ability to verbosely logs all traffic via an environmental variable

[Shaptic]

Is your feature request related to a problem? Please describe.
It can be difficult to debug and root-cause problems in certain environments in which you have less control or access to source code (e.g. containers).

Describe the solution you'd like
Given the presence of an environmental variable, e.g. STELLAR_SDK_VERBOSE=1, all requests and responses should be logged.

Describe alternatives you've considered
This is achievable by modifying the HorizonAxiosClient and adding an interceptor (axios docs), but it is not as convenient as a built-in option.