Add S7 connectivity test

[ingoaf]

Users of the connectivity tester would like to use the app to test a connection to S7 PLC besides the already existing methods.

This feature was tested on an actual edge device (IPC227E) and several S7-300.

This PR:

• adds a backend route for S7 connectivity, performing a simple connection tast via snap7
• adds a frontend page with ip, slot, rack, port input fields
• adds small readme section

[CLAassistant]

All committers have signed the CLA.

[ingoaf]

@Milan-Hurban I signed the agreement, but unfortunately, the check is still pending.

[jpbernius]

Hello @ingoaf,
the problem is that the commits are made with sour Siemens Email, which is not connected to your GitHub Account. So the solution is to add your Siemens Email to your GitHub account as an additional email address.

Additionally, you can join the @siemens organisation (see here), so that you are exempt from the CLA check.

[ingoaf]

@jpbernius good catch, thank you for the information. I have added my work email as suggested, but I did not find a way to resign the CLA. If there is no simpler way (besides joining the Siemens organization) I will reopen the PR and hope that I am able to resign the CLA.

[jpbernius]

I clicked the recheck link. Looks like the check succeeded now.