[Enhancement] Support the MIXED message type

[dingshuangxi888]

Before Creating the Enhancement Request

• I have confirmed that this should be classified as an enhancement rather than a bug/feature.

Summary

We need to introduce a new message type called "MIXED" to support users sending different types of messages on the same topic. This will enable users to send messages of various types through a single topic.

Motivation

Currently, many users are using a single topic to send messages of different types. However, in the future, there is a desire to exert strict control over message types. Message type validation has already been implemented in the Proxy component. To support the transition of existing users to the Proxy architecture, a new message type called "MIXED" should be introduced to facilitate the migration process.

Describe the Solution You'd Like

1. Add MIXED type in TopicMessageType，And modify the validation mechanism.
2. Automatic recognition of message types when sending messages。

Describe Alternatives You've Considered

No.

Additional Context

No response