avro

AVRO-3639: Add fleet.toml and .cargo/config.toml to gitignore

We don't use .cargo/config.toml for anything custom for now!
Using Jetbrains Fleet generates such files with its custom stuff inside.
If we ever need to add custom and general Cargo config then we
(developers) should stop using Fleet.

Signed-off-by: Martin Tzvetanov Grigorov <mgrigorov@apache.org>