Support Native Flink UNIX_TIMESTAMP Function

[Tartarus0zm]

The Flink UNIX_TIMESTAMP UDF is a commonly used user-defined function.
UNIX_TIMESTAMP has multiple usage methods.

• 0 inputs：Gets current Unix timestamp in seconds. This function is not deterministic which means the value would be recalculated for each record.
• 1 inputs：Converts a date time string string1 with default format 'yyyy-MM-dd HH:mm:ss' to Unix timestamp (in seconds).
• 2 inputs：Converts a date time string string1 with format string2 to Unix timestamp (in seconds).

Notes: In Java, the date format is 'yyyy-MM-dd HH:mm:ss', but in Rust it is '%Y-%m-%d %H:%M:%S', to ensure compatibility with Flink scenarios, we need to convert the format.

[weiqingy]

Hi @Tartarus0zm, I can start working on #1863 .

[Tartarus0zm]

Hi @Tartarus0zm, I can start working on #1863 .

hi @weiqingy assigned to you

[weiqingy]

Started on this. Splitting the work into two commits, native first.

The native Flink_UnixTimestamp function (the 1-arg and 2-arg string-parsing forms) is up as #2409.

The Flink converter that emits it is ready too. Since it depends on the native side, I'll open it against this repo once #2409 merges. In the meantime it is previewable here, diffed so it shows only the converter change: weiqingy#1

The 0-argument form (current timestamp, recalculated per record) is not covered yet. It needs different handling from the string-parsing forms, since a 0-arity function has no input column to size the output batch against, and the value has to be per-record to match Flink. I'll keep this issue open to track that remaining piece.