Expose esp_mqtt_client_set_uri. Fix issue #481.

[JakobLachermeier]

Exposes esp_mqtt_client_set_uri in both blocking and async clients.
Fixes #481 by moving unsafe { core::ffi::CStr::from_bytes_with_nul_unchecked(&work.topic) } into their respective commands. I'm not sure why this works.
Doesn't fix #419. I'm also not sure why the original code works on xtensa but not riscv.

[ivmarkov]

Fixes #481 by moving unsafe { core::ffi::CStr::from_bytes_with_nul_unchecked(&work.topic) } into their respective commands. I'm not sure why this works. Doesn't fix #419. I'm also not sure why the original code works on xtensa but not riscv.

I'm also not sure why this works if it works. It might just be masking out a deeper problem we are not seeing, as moving the topic unwrapping inside the legs of the match is equivalent to the previous code (I mean, if we put aside your new SetUri branch which does not consume the topic and where the value of the topic might be invalid of course).

With that said, I'm OK to merge this. When I have to change the URI, I tend to re-create the whole MQTT client and connection, but I guess having set_uri might be more ergonomic for certain use cases.