feat: MySQL Connector (Source + Sink)

[tusharagrahari]

Description

Apache Iggy should support MySQL as a connector for both
Source and Sink.

MySQL is one of the most widely used relational databases.
Supporting it as a connector would allow users to either
ingest data from MySQL into Iggy streams or push Iggy
stream data into MySQL tables.

Affected area / component: Connectors

Proposed solution
Add core/connectors/sinks/mysql_sink and
core/connectors/sources/mysql_source.

I'll start with the Sink implementation first and follow
up with Source as a separate PR.

Expected support:

• Remote MySQL endpoint configuration
• Credential fields stored as SecretString
• Configurable table target
• Insert and upsert modes
• Payload formats: json, text, raw
• Configurable batch size and retry config
• Follow existing connector layout and naming conventions

Note: Will study existing connectors like postgres_sink
as reference and open a discussion if any design questions
come up during implementation.

Affected area / component

Connectors

Proposed solution

No response

Alternatives considered

No response

Contribution

• I'm willing to submit a pull request to implement this feature

Good first issue

• I think this could be a good first issue for a new contributor