feat: adds support for audience in client_options in v1 branch

[parthea]

feat: add api_key to client options in v1 branch

Cherry-pick #248 and #379 into v1 branch

These features are needed so that we can continue to support google-api-core 1.x in cloud client libraries.

Towards b/238481913

[parthea]

Unit tests pass locally for python 3.7 and 3.9

partheniou@partheniou-vm-2:~/git/python-api-core$ nox -s unit-3.9
nox > Running session unit-3.9
nox > Creating virtual environment (virtualenv) using python3.9 in .nox/unit-3-9
nox > python -m pip install mock pytest pytest-cov 'grpcio >= 1.0.2'
nox > python -m pip install -e . -c /usr/local/google/home/partheniou/git/python-api-core/testing/constraints-3.9.txt
nox > python -m pip install asyncmock pytest-asyncio proto-plus
nox > python -m py.test --quiet --cov=google.api_core --cov=tests.unit --cov-append --cov-config=.coveragerc --cov-report= --cov-fail-under=0 tests/unit --cov=tests.asyncio tests/asyncio
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 partheniou@partheniou-vm-2:~/git/python-api-core$ nox -s unit-3.7
nox > Running session unit-3.7
nox > Creating virtual environment (virtualenv) using python3.7 in .nox/unit-3-7
nox > python -m pip install mock pytest pytest-cov 'grpcio >= 1.0.2'
nox > python -m pip install -e . -c /usr/local/google/home/partheniou/git/python-api-core/testing/constraints-3.7.txt
nox > python -m pip install asyncmock pytest-asyncio proto-plus
nox > python -m py.test --quiet --cov=google.api_core --cov=tests.unit --cov-append --cov-config=.coveragerc --cov-report= --cov-fail-under=0 tests/unit --cov=tests.asyncio tests/asyncio
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