FuriHal: Serial hardware flow control?

[WillyJL]

What's new

• @bettse brought up on the discord server the topic of UART hardware flow control, in specific how the GPIO app USB-UART allows choosing RTS/DTR pins but the code doesn't seem to affect the UART pins behavior
• personally I have no experience with this topic and don't understand it much, but as bettse pointed out the GPIO side in FuriHal is initialized with HardwareFlowControl = LL_USART_HWCONTROL_NONE so I thought exposing this as an option to API would be a good start
• neither of use have anything to test this on and I don't have a use for it either, just putting it here in case it makes sense and someone else could find it useful and make it work
• also to note, the USB-UART app mentions RTS/DTR while the stm32 hal has RTS/CTS options, the difference isn't really clear to either of us

Verification

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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

[GMMan]

IIRC RTS is used in special ways for programming some microcontrollers, such as ESP8266, so if you have one of those laying around, you could test parts of it.