Add support for .NET verification on installer launch.

[freakboy3742]

Adds support for confirming a specific .NET runtime version is available before launch.

Refs beeware/briefcase#2804.

briefcase-repo: https://github.com/freakboy3742/briefcase.git
briefcase-ref: dotnet-verify

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

Seeing a lot of apparently random failures on Windows runners at the moment:

• https://github.com/beeware/toga/actions/runs/24858249599/attempts/1
• https://github.com/beeware/briefcase-windows-VisualStudio-template/actions/runs/24858054025/attempts/1
• https://github.com/beeware/briefcase-windows-VisualStudio-template/actions/runs/24858882581/attempts/1

[freakboy3742]

Seeing a lot of apparently random failures on Windows runners at the moment:

Hrm... concerning, but they're all passing on a re-run... and some of them (like the first one) look like it's a Github problem...