chore: add a compile cfg for python in cmd package

[discord9]

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What's changed and what's your intention?

add a "python" flag in cmd package so can turn off python feature when building greptime binary i.e. cargo build --release --bin greptime --no-default-feature, note the --no-default-feature which turn off default features which in turn turn off python featue, since the only default features in cmd is python

LIMITATION: user who want to turn off python features might need to also figure out is there any features they want to keep because they need to pass --no-default-features first to turn off all default features

Checklist

• [ ] I have written the necessary rustdoc comments.
• [ ] I have added the necessary unit tests and integration tests.
• [ ] This PR requires documentation updates.