refactor: remove associate type in scheduler and handler to simplify flush and compaction #2153

[Nateiru]

…flush and compaction

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

• Flush and Compaction processes involve the Request associative type, which is polymorphic overhead leveraging static dispatching when compiled.
• I delete the Request associative type and replace it with dyn Reqeust.
• This design aims to reduce polymorphic overhead leveraging static dispatching. But this overhead is negligible in this case.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

closes #2153