feat: support `explain` syntax

[francis-du]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

• [ ] Add explain to sql parser
• [ ] Add explain test to sql parser
• [ ] Handle explain in DataNode instanse
• [ ] Handle explain in Frontend instanse

Checklist

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

Refer to a related PR or issue link (optional)

Closes #531