In order to integrate into the Envoy build system better, the bssl-compat build system must be converted to bazel.

This will speed up the edit-compile-debug cycle time for local dev work on bssl-compat, because instead of appearing to bazel as a single black box CMake build, it should expose much finer grained dependencies.

Also, this is a prerequisite for enabling the bssl-compat layer to be built using the bazel-fetched BoringSSL, rather than having it's own vendored copy of BoringSSL (OSSM-1207).