Dropbox Client uses wrong httpclient which causes port exhaustion

[evolvedlight]

Describe the bug
When using the Dropbox client in high traffic multithreaded environment, it's possible to easily exhaust all connections

To Reproduce
In a loop, keep creating Dropbox Clients and performing a request. In the options object, pass in a HTTPClient.

Expected Behavior
HttpClient is used for all requests

Actual Behavior
Authentication code makes a new HttpClient each time

Versions

• SDK latest, 6.31.0
• Dotnet 6
• Linux kubernetes

Additional context
It's also difficult to see what the recommended way is to reuse Dropbox clients - ie it's not very threadsafe but it's also bad to keep creating new Dropbox Clients

[greg-db]

Thanks for the report and PR!