[Feature Request]: Make Exec task support new progress messages.

[AR-May]

Summary

Design and implement a (limited) progress messages support in the Exec task.

Background and Motivation

With the new progress messages API #10415, custom task authors would be able to have an improved logger experience. However, there are many builds that use the Exec task in order to execute some script, instead of writing a custom task. It would be beneficial to have some support for the new progress messages API for the Exec task users as well.

Proposed Feature

Similar to CustomErrorRegularExpression and CustomWarningRegularExpression, add CustomProgressRegularExpression to the Exec task parameters to provide a detection of messages that should be forwarded through the new progress messages API.

Alternative Designs

Up for a discussion.

[selcukgural]

I've opened a PR in dotnet/aspnetcore (#62273) to surface generated OpenAPI document paths via instead of relying on stdout. This way the output goes through MSBuild's message infrastructure.

However, full visibility at default Terminal Logger verbosity still depends on this issue being resolved — MessageImportance.High messages are currently not shown by Terminal Logger unless -tlp:v=d is used.

Once this is fixed on the MSBuild side, the aspnetcore change will work end-to-end without any additional changes needed.