Update P4 C++ API to 2021.2/2201121 and PerforcePlugin infrastructure

[SRombautsU]

VCS-1007369 [PerforcePlugin] rebuild with P4 API 2021.2/2201121 and test with Perforce Cloud

Simplify, fix infrastructure, and document how the P4 C++ API are downloaded and linked into the PerforcePlugin

"Your client does not support Helix Core Cloud. Upgrade to version 2021.2/2201121 or later"
Perforce plugin unusable with a Perforce Helix Core Cloud server #23

When trying to connect to our newly set up Perforce Helix Core Cloud server I get the following error: "Your client does not support Helix Core Cloud. Upgrade to version 2021.2/2201121 or later". The server then rejects the connection completely, making the Perforce plugin unusable. From what I can tell, the version of the P4 API included in the plugin right now is still 2019.1.

While troubleshooting I found a community maintained updated version of the Perforce plugin in a repo over on the Perforce GH org: https://github.com/perforce/unity-p4-plugin. This alternative plugin has worked for me but requires overwriting the plugin executable included in the Unity Editor install directory, which is not ideal. It also seems to not be under the same license so maybe that's why they didn't want to upstream the changes? Not sure.

In any case it would be good if the P4 API was updated here so that Helix Core Cloud servers would be supported OOTB.

The only change in the code itself is that now the plugin has a version number, and log it and the P4 API:

UNITY > c:pluginConfig end
Got config end = ''
PerforcePlugin 1.2.1
Client API: 86
Client Build: P4API/NTX86/2019.1/2135798
Perforce password is empty. Ignoring login request.
v1:'login' not necessary, no password set for this user.
Is logged in: yes
spec::Run()
v1:client -o "unity-perforce_3G4NJ84" 

From these more solid foundations, I'll want to update one thing without changing anything, and testing if it still works as expected, then moving to the next (stopping when it breaks or doesn't bring any more value):

1. (this task) add a version to the plugin, and log them
2. (this task) cleanup Perl build and test scripts
3. (this task) fix CI to fail on failures (!), and disable login2 failing test
4. (this task) update the API from r19.1 to 2021.2 (the minimum for cloud support) to 2025.1 (the latest)
5. (future) update Visual Studio from 2010 to 2019 (max for p4api 2021.2)
6. (future) update the OpenSSL lib from deprecated 1.0.1 to 1.1.1w (eol 2023) to 3.x (the only one supported)
7. (future) update the API to 2023.2 (the first with support VS2022)
8. (future) update Visual Studio to 2022 (the only one supported) - require p4api 2023.2
9. (future) update the API to 2024.1 (latest community supported)?
10. (future) update the API to 2025.1 (FAILED)