Make slash command output streamable (#19632)

This PR adds support for streaming output from slash commands

In this PR we are focused primarily on the interface of the
`SlashCommand` trait to support streaming the output. We will follow up
later with support for extensions and context servers to take advantage
of the streaming nature.

Release Notes:

- N/A

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Co-authored-by: David Soria Parra <davidsp@anthropic.com>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: David <david@anthropic.com>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Will <will@zed.dev>