feat(udf): add body field for udf body definition

[xzhseh]

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What's changed and what's your intention?

As titled.
This branch is based on #14139, just for review convenience.

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[yezizp2012]

Rest LGTM.

[xzhseh]

If it looks good now I'll merge it, cc @yezizp2012 @wangrunji0408.

[yezizp2012]

May also change it as optional in proto and function catalog definition. Rest LGTM.