Remote source file cache

[ViktorHofer]

With 1463ade the functionality to fetch remote files was implemented. We are using ReportGenerator to get comprehensive reports for fundamental parts of .NET Core like System.Private.CoreLib and other assemblies. As we don't build System.Private.CoreLib ourselves and can't resolve source code paths contained in the pdb, we use sourcelink and tell coverlet to forwards these remote paths into the generated report (as expected).

The issue that we are hitting now is that fetching these remote files takes a considerable amount of time and generating the report with ReportGenerator now takes multiple minutes. We should implement a remote source file cache as these paths don't change very often when using sourcelink.

@danielpalme please let me know if you are fine with the idea and want me to send a PR.

cc @stephentoub

[danielpalme]

Makes sense.
We should define a setting to configure the caching duration.
If a file is older than this timespan, it get's downloaded again.

I can do the implementation too.

[ViktorHofer]

In the case of source link the remote URL is already unique as it points to a specific commit. Adding a timeout wouldn't be necessary for that use case.

[ViktorHofer]

I think placing the cache in a temporary location that gets cleaned up by the operating system frequently should suffice for us. Of course I understand that a timeout could make sense for other scenarios.

[ViktorHofer]

If we add a parameter to enable/disable the cache then I guess we don't need a timeout. Daniel if you have time it would be great if you could implement it 👍

[danielpalme]

New release is available:
https://www.nuget.org/packages/ReportGenerator/4.0.7
https://www.nuget.org/packages/dotnet-reportgenerator-cli/4.0.7
https://www.nuget.org/packages/dotnet-reportgenerator-globaltool/4.0.7

Files are cached for one week and then downloaded again.
The caching duration can be changed in the settings.