Add MQTTnet package

[TapioRantala]

The NuGet package needs to respect a few constraints in order to be listed in the curated list:

• Add a link to the NuGet package: https://www.nuget.org/packages/MQTTnet/
• It must have non-preview versions (e.g 1.0.0 but not 1.0.0-preview.1)
• It must provide .NETStandard2.0 assemblies as part of its package
• The lowest version added must be the lowest .NETStandard2.0 version available
• The package has been tested with the Unity editor
• The package has been tested with a Unity standalone player
  • if the package is not compatible with standalone player, please add a comment to a Known issues section to the top level readme.md
• All package dependencies with .NETStandard 2.0 target must be added to the PR (respecting the same rules above)
  • Note that if a future version of the package adds a new dependency, this dependency will have to be added manually as well

Note: The server will be updated only when a new version tag is pushed on the main branch, not necessarily after merging this pull-request.

The version is not the first version available on .NETStandard2.0, that would be 2.5.1.
There are additional dependencies listed for versions before 3.0.15:
NETStandard.Library (>= 2.0.0)
System.Net.Security (>= 4.3.2)
System.Net.WebSockets (>= 4.3.0)
System.Net.WebSockets.Client (>= 4.3.1)
System.Threading.Thread (>= 4.3.0)
These look like they are part of the .NETStandard2.0 itself?

Not sure if I should add those with "ignore": true or if having not-lowest-version is ok.

[bdovaz]

@TapioRantala

2023-10-06T12:33:32.4929492Z
2023-10-06T12:33:40.7789869Z Failed Ensure_Min_Version_Is_Correct_Ignoring_Analyzers_And_Native_Libs [11 s]
2023-10-06T12:33:40.7799156Z Error Message:
2023-10-06T12:33:40.7805032Z Package MQTTnet
2023-10-06T12:33:40.7813637Z Expected: <2.5.1>

[bdovaz]

@TapioRantala please, run tests locally

Failed Ensure_That_Packages_Already_Included_In_Net_Standard_Are_not_Included_In_The_Registry [140 ms]
2023-10-09T12:19:10.5023362Z Error Message:
2023-10-09T12:19:10.5029297Z Expected:
2023-10-09T12:19:10.5032267Z But was: < "System.Net.Security", "System.Net.WebSockets", "System.Threading.Thread" >
2023-10-09T12:19:10.5032694Z
2023-10-09T12:19:10.5032811Z Stack Trace:
2023-10-09T12:19:10.5033519Z at UnityNuGet.Tests.RegistryTests.Ensure_That_Packages_Already_Included_In_Net_Standard_Are_not_Included_In_The_Registry() in D:\a\UnityNuGet\UnityNuGet\src\UnityNuGet.Tests\RegistryTests.cs:line 3

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[TapioRantala]

I believe the concerns have been addressed now and this is ready to be merged.

[bdovaz]

Please, check comment @TapioRantala

[TapioRantala]

Tests are passing, please clarify.