Add route string syntax for pattern in RouteHandlerServices.Map

[murshex]

{PR title}

• You've read the Contributor Guide and Code of Conduct.
• You've included unit or integration tests for your change, where applicable.
• You've included inline docs for your change, where applicable.
• There's an open issue for the PR that you are making. If you'd like to propose a new feature or change, please open an issue to discuss the change or find an existing issue.

Summary of the changes (Less than 80 chars)

Description

Add route string syntax for pattern in RouteHandlerServices.Map. Just that.

Fixes #59939

[dotnet-policy-service]

Thanks for your PR, @murshex. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.