fix: add method to close httplib2 connections

[busunkim96]

Fixes #618 🦕

httplib2 leaves sockets open by default. This can lead to situations where programs run out of available sockets. httplib2 added a method last year to clean up connections. https://github.com/httplib2/httplib2/blob/9bf300cdc372938f4237150d5b9b615879eb51a1/python3/httplib2/__init__.py#L1498-L1506

This PR adds two ways to close http connections. The interface is intentionally similar to the proposed design for GAPIC clients. googleapis/gapic-generator-python#575

explicitly call close:

from googleapiclient import discovery

service = discovery.build("drive", "v3")
service.drives().list()
service.close()

context manager:

from googleapiclient import discovery

with discovery.build("drive", "v3") as service:
    service.drives().list()

# service.__exit__() is called which calls service.close()

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