Movments - #1

This pull request adds support for handling generic data packets sent from the server to the client via the data channel. It introduces a new on_data event callback, updates the relevant types and documentation, and ensures that payloads are base64-decoded before being delivered to the handler.

Event and Data Handling Enhancements:

• Added a new optional on_data callback to the EventHandler class, which receives topic-addressed data packets from the server with payloads already base64-decoded. This allows clients to handle fire-and-forget server data, such as locomotion commands, in a structured way. (src/streamcore/types.py [1] README.md [2] README.zh-CN.md [3]
• Updated the DataChannelMessage type to support messages of type "data", including new topic and payload fields for carrying the data packet information. (src/streamcore/types.py src/streamcore/types.pyL60-R68)

Implementation Details:

• Modified the message handling logic in StreamCoreClient to decode base64 payloads from "data" messages and invoke the on_data callback if set, with error handling for malformed payloads. (src/streamcore/client.py src/streamcore/client.pyR503-R511)
• Added necessary imports for base64 and binascii to support payload decoding. (src/streamcore/client.py src/streamcore/client.pyR4-R5)

Project Versioning:

• Bumped the package version to 0.1.5 to reflect the new feature. (pyproject.toml pyproject.tomlL7-R7)