Create a bazel build element

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/529)
In GitLab by [[Gitlab user @edbaunton]](https://gitlab.com/edbaunton) on Jul 30, 2018, 16:20

## 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.

Bắt đầu với GitLab issue được liên kết cùng phần mô tả Background và Task của issue đó. Hãy lập bản đồ các entry point hiện có của phần tử BuildStream trước khi thiết kế việc thực thi Bazel trong sandbox, xử lý dependency đệ quy, tạo WORKSPACE, mount persistent cache và tạo toolchain cho target platform; công việc được xem là hoàn tất khi các yêu cầu này được hỗ trợ cùng nhau.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.