Implement license validation for project dependencies

[ivankravets]

A project can depend on multiple 3rd party packages: libraries, frameworks, SDKs. To help companies avoid license risks, PlatformIO can analyze licenses of packages that are used for the build process and alert users if there is a conflict.

• License Compatibility.pdf
• https://spdx.github.io/spdx-spec/SPDX-license-expressions/