Allow to create ParticipantSid from String in user module (#250)

[trzeciak]

Hey, I want test/use this livekit-rust-sdk in c++ code.
To do this I create internal-rust-layer to expose some functionality, and extract it to c++.
I came across a problem to create ParticipantSid from String in user module,
therefore i purpose to add 'pub' in declaration of ParticipantSid.
Do you think it is ok, or I should choose other way?

[CLAassistant]

All committers have signed the CLA.

[theomonnom]

Hey, to ensure type safety of ParticipantSid, the String must always start with PA_. This is why you can't directly create from a String.
Instead you can do ParticipantSid::try_from(my_string).unwrap()

[trzeciak]

Thanks for the explanation, I will follow it 👍