feat: add vec_kth_elem function

[Pikady]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

#5120

What's changed and what's your intention?

add vec_ktm_elem function: Returns the k-th element of the vector.

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

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

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[zhongzc]

It would be more appropriate for kth_elems to use 0-based indexing.

[zhongzc]

Please fix CI

[zhongzc]

There are some merge conflicts that need to be resolved, can you fix them?

[Pikady]

There are some merge conflicts that need to be resolved, can you fix them?

I've resolved the merge conflicts.