Update import document tool to new auth model

[gitri-ms]

Motivation and Context

The recent changes to the authentication model in Chat Copilot effiectively broke the import document tool. This change fixes it, by enabling users to:

• Use the tool with an unauthenticated local instance of Chat Copilot without signing in
• Use the tool with an Azure-deployed instance of Chat Copilot with Azure AD authentication enabled

Fixes #212, fixes #231

Description

• Adds code paths to authenticate user (or not) depending on configuration
• Removes user name/info from document import form
• Updates README with instructions for setting up the app registration for import document tool and configuring appsettings.json

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible - N/A, but both supported scenarios have been tested
• I didn't break anyone 😄