[Feature Request] Improve the experience with long running process in web APIs leveraging MSAL.NET new API

[jmprieur]

Is your feature request related to a problem? Please describe.

• Today, the developer experience with long running process is described in https://github.com/AzureAD/microsoft-identity-web/wiki/get-token-in-event-handler
• MSAL introduces new public APIs to support this scenario better. See issue [Feature Request] Allow customers to provide a key to access the OBO token for long running process microsoft-authentication-library-for-dotnet#2733 and PR Add public API for long-running OBO methods with custom cache key and do not use refresh token for normal OBO microsoft-authentication-library-for-dotnet#2820.

Describe the solution you'd like

• Option 1: Surface the verbs in ITokenAcquisition or rather an extension class of ITokenAcquisition to avoid a breaking changes
• Option 2: Have an interface derived from ITokenAcquisition
• Option 3: add a longRunningProcessSessionKey in TokenAquisitionOptions.

[jennyf19]

Included in 1.21.0 release