Create a bazel build element

Background

Create an element that provides support for building with bazel.

Specifically, a bst would declare itself of kind bazel and BuildStream would take care of generating the WORKSPACE file by recursively scanning the dependencies of the element being built.

Task description

• bazel would be executed inside the sandbox (not using a host tool)
• The workspace file would identify the built sources to consume with new_local_repository rules; if it encounters a dependency of type bazel it would stage the sources rather than the binaries and consume the BUILD file to leverage bazel.
• bazel would need access to a persistent disk cache (use _set_mount_source?) between BuildStream runs so as to take advantage of its built in caching. The bazel server will actually be torn down between runs but it is said to serialize its state between runs so the startup time will no be prohibitive.
• Toolchain generation: we need to generate correct toolchain for the target platform and have that be considered as part of the key for the element.