[FL-3927] FuriThread stdin

[portasynthinca3]

What's new

• Support for stdin in threads
• Support for additional context for stdout and stdin callbacks
• Unit tests for stdin and already existing stdout functionality
• Small fix in furi_string docs

Verification

• Run unit tests

Checklist (For Reviewer)

• PR has description of feature/bug or link to Confluence/Jira task
• Description contains actions to verify feature/bugfix
• I've built this code, uploaded it to the device and verified feature/bugfix

[github-actions]

Compiled f7 firmware for commit 53fc736a:

• 📦 Update package
• 📥 DFU file
• ☁️ Web/App updater
• 📊 Size report

[CookiePLMonster]

What is the intended use of this? Can ufbt cli capture inputs into that stdin?

[portasynthinca3]

What is the intended use of this? Can ufbt cli capture inputs into that stdin?

@CookiePLMonster The intended use case is to run commands in separate threads. For that, threads have to have a stdout (already in place) and a stdin (implemented in this PR).