Added .NET implementation to docs for streaming subscription support

[WhitWaldo]

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Description

Added an example for implementing streaming subscription support in .NET.

Please do not merge this until this PR has been accepted and merged

Issue reference

It won't necessarily close either, but it's associated with this PR and this issue.

[WhitWaldo]

/assign

[WhitWaldo]

/assign

[github-actions]

Stale PR, paging all reviewers

[WhitWaldo]

Despite being marked as stale by the bot, please do not merge this until this PR has been accepted and merged, per the ask in the description.

[github-actions]

Stale PR, paging all reviewers

[WhitWaldo]

Blocked pending on dapr/dotnet-sdk#1380 now instead of dapr/dotnet-sdk#1346

Just kidding - blocked on dapr/dotnet-sdk#1381 instead.