RFE: Add `bst validate` command

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/638)
In GitLab by [[Gitlab user @cs-shadow]](https://gitlab.com/cs-shadow) on Sep 10, 2018, 13:32

## Background

This command would validate project, element and optionally user configurations. It will be useful to verify that all configs actually are valid before trying to build them. It will be especially useful for running under CI as a lightweight check for verifying the configs.

At the very least, this command would provide a yes/no kind of feedback for given config files, but ideally it may also try to detect common anti-patterns and report them.

## Task description

[//]: # (Short summary of the action to be executed)

* [ ] Add a top-level `bst` command to validate `*.bst` files

## Acceptance Criteria

[//]: # (Acceptance criteria should follow the S.M.A.R.T. principle https://en.wikipedia.org/wiki/SMART_criteria )

One is able to verify all element and project configuration using a single command.

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The issue names no files or tests. Start by locating the top-level `bst` command entry point and the existing validation paths for project, element, and optional user configuration; done means one command can validate all configurations and provide yes/no feedback.

Written by the indexing model from the issue text.