Adding option for plugin timeout + using proper userId for guest accounts

[teresaqhoang]

Motivation and Context

Two small changes in this PR:

1. Using ${account.localAccountId}.${account.tenantId} over account.homeAccountId to match how UserId is constructed for chat messages in webapi.
2. Adds another option in appsettings to allow users to set a timeout limit for plugin calls.

Description

1. How userId is constructed in webapi:
   

• ${account.localAccountId}.${account.tenantId} is more accurate for guest accounts, and it is not equivalent to the homeAccountid (which = {userIdInHomeTenant}.{HomeTenantId} for guest accounts).

2. Default HttpClient.Timeout is 100 seconds; customer request to allow this to be configurable to accomodate plugins that take a long time.

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
• I didn't break anyone 😄