Add FileDiagnosticLogger to assist with debugging the SDK

[mattjohnsonpint]

Sometimes when debugging, I want to capture the log output to a file. Copy/paste from console isn't always possible (depending on app type), and even when it is, It could be mixed with other console output, or it could be truncated due to size of the console buffer.

This adds a FileDiagnosticLogger that writes diagnostic info to the specified file. For example:

    options.Debug = true;
    options.DiagnosticLogger = new FileDiagnosticLogger("log.txt");

Optional parameters can be used to set log level, or to write to both file and console simultaneously.

[github-actions]

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

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[vaind]

worth updating https://docs.sentry.io/platforms/dotnet/guides/aspnet/configuration/diagnostic-logger/ too?

[mattjohnsonpint]

Docs PR staged here:

getsentry/sentry-docs#6493

[bruno-garcia]

The docs PR never got merged: getsentry/sentry-docs#6493

[mattjohnsonpint]

It will after this is released. 😉

[bruno-garcia]

oh I saw the message above was 2 weeks ago, assume the code was live for ages