Merge pull request #9776 from apolcyn/add_http2_flow_control_interop_tests

doc/http2-interop-test-descriptions.md

test/http2_test/http2_base_server.py

test/http2_test/http2_test_server.py

test/http2_test/test_data_frame_padding.py

tools/doxygen/Doxyfile.c++

tools/doxygen/Doxyfile.c++.internal

tools/doxygen/Doxyfile.core

tools/doxygen/Doxyfile.core.internal

tools/internal_ci/linux/grpc_interop_badserver_java.sh

tools/internal_ci/linux/grpc_interop_badserver_python.sh

tools/jenkins/run_interop.sh

tools/run_tests/interop/interop_html_report.template

tools/run_tests/python_utils/report_utils.py

tools/run_tests/run_interop_tests.py