refactor(sink): DRY SinkFormatterImpl::new

[xiangjinwu]

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What's changed and what's your intention?

Split the monolithic SinkFormatterImpl::new into extensible EncoderBuild & FormatterBuild pieces. The dispatcher logic in SinkFormatterImpl::new is now simply matching from (format, encode) into a variant of the SinkFormatterImpl enum.

This is NOT a purely equivalent refactor. By reducing the repetition, it also:

• supported schemas.enable option for format plain encode json, which was only available for format upsert encode json #12113.
• fixed partition skew problem when encode template is used with format plain without key. The encode json case was fixed in #12810.

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[tabVersion]

Thanks for the refactor. It helps a lot with adding new sink format impl combinations 🫡