Allow async ImapFetchStreamCallback for ImapFolder.GetStreams methods

[lkstz]

Is your feature request related to a problem? Please describe.
When downloading multiple messages using one of the ImapFolder.GetStreams overloads, it is currently not possible to use an async method as ImapFetchStreamCallback. Many .NET APIs are async-first if not async-only nowadays, so one has to build complex workarounds to allow async processing of the messages.

Describe the solution you'd like
I'd like to have an async version of ImapFetchStreamCallback, e.g. ImapFetchStreamAsyncCallback:

public delegate Task ImapFetchStreamAsyncCallback (ImapFolder folder, int index, UniqueId uid, Stream stream);

Each ImapFolder.GetStreams method would get an overload accepting an ImapFetchStreamAsyncCallback.

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If this feature is not too complicated, I'd be open to implement it and make a PR.

[jstedfast]

Hmmm, yes, this should have been an async callback. I must have missed this when I rewrote MailKit to be async from the ground up in the 2.0 rewrite.

I think the solution for now is probably to fix the GetStreamsAsync() methods to take an ImapFetchStreamAsyncCallback delegate instead of duplicating the API (which I really hate doing).

I'll leave the GetStreams() APIs alone, so they will continue to use the old callback delegate (which probably makes sense anyway since they probably don't care about being async).

In a future version, I'll change the ImapFetchStreamCallback delegate signature to also take a CancellationToken argument like I'm adding to the ImapFetchStreamAsyncCallback delegate signature.

[lkstz]

👍 Thank you for the fast reaction!