Refactor Python dependencies into multiple files depending on app

[rdimaio]

Description

To meet this request: #6663

We agreed to split the single requirements.txt file into multiple files. For the client, the dependencies should be unpinned (as the client is used as a library in certain use cases), whereas we should keep all dependencies pinned for the server.

Motivation

No response

Change

No response

[rdimaio]

For historical context - the dependencies were initially in separate files, but they were collected in a single requirements.txt file as part of this issue: #4456

[rdimaio]

Could be worth using pip-compile to auto-generate pinned dependencies: https://github.com/jazzband/pip-tools

More info here: https://stackoverflow.com/questions/66751657/what-does-pip-compile-do-what-is-its-use-how-do-i-maintain-the-contents-of-my