Build upstream Envoy against the bssl-compat library.

The envoyproxy/envoy-openssl repository includes the bssl-compat library, in the bssl-compat subdirectory, and the upstream envoyproxy/envoy repository as a submodule.

When using bazel to build from the top level, it should build the bssl-compat library and then build envoy using the bssl-compat library instead of BoringSSL.

Success Criteria:

• Envoy should be compiled against the bssl-compat headers (with compilation errors depending on bssl-compat progress).
• Envoy should be linked against the bssl-compat library (with unresolved symbols depending on bssl-compat progress).
• The `.github/workflows/envoy.yml` workflow should run the envoy build (Best not to do this because of the size of the envoy build! I think we only get 2000 free minutes/month for github actions)