Support multiple entries in explorer for a single start page

## Problem
A user has to constantly switch the connection in explorer if the user is using multiple accounts. If a user has separate start pages then multiple would occur. It is possible to 'switch connections' but it would be better to have multiple account entries in explorer.

## Expected behavior
1. Right click existing explorer connection
2. Select "Add New Connection"
3. Select "IAM Identity center"
4. In "Enter Start URL" enter the exact same start ID for an existing account in the same start page and DO NOT say "A connection for this start url already exists" Support a secondary account at the same start URL

Start from the VS Code Explorer connection flow described in the issue, especially “Add New Connection” for IAM Identity Center and its existing-start-URL check. Trace how connections are keyed by the start URL; done means a second account can be added at the same start URL without the duplicate warning and both accounts appear in Explorer.

Written by the indexing model from the issue text.