[Test Proxy] Add environment_variables fixture

[mccoyp]

Description

This introduces an environment_variables fixture that emulates today's EnvironmentVariableLoader. This fixture provides an interface for easily fetching and sanitizing environment variables, and can either be used on a test-by-test request basis, or on a module/session basis to prepare test variables for all tests.

For example, a test file could do something like the following to sanitize test variables while making them available to all tests, without duplicating code:

import pytest
from devtools_testutils import AzureRecordedTestCase

test_variables = {}

def prepare_test_variables(environment_variables):
    sanitizings = {
        "SERVICE_VARIABLE_A": "fake_value_a",
        "SERVICE_VARIABLE_B": "fake_value_b",
        "SERVICE_VARIABLE_C": "fake_value_c",
    }
    test_variables = environment_variables.sanitize_batch(sanitizings)

@pytest.mark.usefixtures("recorded_test")
class TestClass(AzureRecordedTestCase):
    def test_method(self):
        result = do_thing(test_variables["SERVICE_VARIABLE_A"])

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