Usability improvements

This release will focus on making the library easier to use. At least the following should be done:

• Update the readme and maybe add some more use cases.
• Have a reliable and faster ci, likely by moving to Github actions.
• Implement the standard Error trait for SRCError.
• For protobuf, have an easier way to encode with single message schema's, not requiring providing the full_name.
• For async, have either an example or in the library a nice way to share the converter in multiple threads, so the users don't have to think about this. See https://github.com/gklijs/ksqlDB-GraphQL-poc/blob/main/rust-data-creator/src/data_producer/kafka_producer.rs but improve on that. I would be nice to not have to depend on rdkafka through.
• Enable supplying a reqwest client for any additional setting/modifications needed for the schema registry calls.