Move out metastore_browser from airflow.contrib

[mik-laj]

Formerly the core code was maintained by the original creators - Airbnb. The code that was in the contrib package was supported by the community. The project was passed to the Apache community and currently the entire code is maintained by the community, so now the division has no justification, and it is only due to historical reasons.

We plan to deprecate the airflow.contrib package. Unfortunately, there are still a few modules left that block it.

In addition, I wrote a short README. which describes how to install this plugin. In the future, we can add more text to this file if we decide to develop this plugin

It's not breaking change, because this plugin was not automatically installed. Anyone who wanted to install it had to manually copy these files to plugin directory. We also did not mention this plugin in any documentation. We did not actively develop it, but rather treated it as an example plugin.

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[potiuk]

You need to include "metastore_browser" in .dockerignore. By default we are excluding everything in .dockerignore and only include what we need (that's the best practice for Dockerignore - makes context to send much smaller when you have a lot of automatically generated artifacts in sources - which we have)