[AURON #1549] Implement native function of initcap.

[slfan1989]

Which issue does this PR close?

Closes #1549.

Rationale for this change

• Align our SQL string functions with Apache Spark by adding initcap(str).
• Provide a native for better performance on large columns.
• Ensure Unicode-aware, locale-independent behavior consistent with Spark semantics.

What changes are included in this PR?

• Implement Unicode-aware word capitalization: first letter uppercase, remaining letters lowercase; non-letters preserved as delimiters.
• Handle null/empty inputs (initcap(NULL) → NULL, initcap('') → '').
• Wire up function registration and planner mapping so INITCAP resolves to the native kernel.
• Add comprehensive unit tests (ASCII, Unicode, delimiters, mixed case, nulls, empties, long strings).

Are there any user-facing changes?

• New function available: initcap(str).

How was this patch tested?

Junit Test & CI.

[guixiaowen]

LGTM