[api] Use Locale.ROOT in DLFOpenApiV4Signer

[taoran92]

Purpose

Fixes #10086. Follow-up to #9770 and #9771.

Canonicalize DLF OpenAPI V4 signing header names with Locale.ROOT. Under a Turkish default locale, X-ACS-SIGNATURE-NONCE otherwise becomes x-acs-sıgnature-nonce, changing the canonical request and signature. The default authentication path generates lowercase headers and is unaffected.

Tests

Added DLFOpenApiV4SignerTest#testAuthorizationWithTurkishLocale, which fixes the timestamp and nonce and compares the complete authorization string for the same mixed-case header under US and Turkish locales. The test restores the original locale in a finally block.

Confirmed that the regression test fails without the fix and passes with it. All 33 tests passed with the standard Maven checks enabled:

mvn -pl paimon-api -DwildcardSuites=none \
  -Dtest=DLFOpenApiV4SignerTest,DLFRequestSignerTest test

[JingsongLi]

This fixes a real signature-invariance issue in the public DLF OpenAPI V4 signing path: an uppercase I in a mixed-case X-ACS-* header becomes a Turkish dotless ı under that JVM default locale, changing the canonical header list and authorization value. Locale.ROOT is the correct normalization and matches the existing V1 signer. The default generated lowercase-header path remains unaffected; I found no regression in this one-line change.

The fixed-timestamp/nonce regression compares the complete authorization value across US and Turkish locales and restores the original locale. I ran DLFOpenApiV4SignerTest and DLFRequestSignerTest locally with standard Maven checks: 33/33 passed. The PR head's JDK, Flink, Spark and E2E CI checks also pass.