Support Bazel as execution client

Context

BuildGrid, even if related to the BuildStream project, is meant to be a generic Remote Execution server and thus, should support Bazel as a Remote Execution client.

Task Description

First step would consist in improving and fixing the existing BuildGrid server and bots in order to have simple builds, using host tools, working.

A second step would be to investigate the sandboxed builds, and how we can support executing Bazel build using custom toolchains distributed as Docker images.

Acceptance Criteria

First stage, simple builds:

• Build Bazel's cpp-example stage1: simple single-file C++ executable.
• Build Bazel's cpp-example stage2 and stage3: simple C++ executable + library.