feat: support Instant string conversion

[56wj]

Purpose of the pull request

Related: #1017

Add JDK 8-compatible support for lossless conversion between java.time.Instant and ISO-8601 Excel string cells.

What's changed?

• Add InstantStringConverter using Instant.parse() for reads and Instant.toString() for canonical writes.
• Register the converter for default writes, explicit string writes, and string reads.
• Add unit coverage for converter keys, UTC and offset parsing, nanosecond round trips, invalid input, and loader registration.
• Keep the implementation string-only because Excel date/number cells do not preserve timezone/offset semantics.

Verification

• mvn clean package -B -Dmaven.test.skip=false -pl fesod-common,fesod-shaded,fesod-sheet
• mvn -pl fesod-sheet -DskipTests spotless:check

Results after rebasing onto current main on JDK 21: 923 tests, 0 failures, 0 errors, 0 skipped. The generated Surefire reports include 6 passing InstantStringConverterTest cases and 5 passing DefaultConverterLoaderTest cases.

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

[56wj]

CI on JDK 8/11 exposed a runtime compatibility difference: Instant.parse(...) rejected the +08:00 test input there, although newer JDKs accepted it. I updated the read path to OffsetDateTime.parse(...).toInstant(), which preserves the intended ISO-8601 offset normalization and remains JDK 8 compatible. The focused converter/loader tests and Spotless check pass locally on the updated commit 87ce5b2.

[bengbengbalabalabeng]

The newly added files in this PR are implemented from scratch and are not derived from Alibaba's EasyExcel. Therefore, no EasyExcel-related license header is required for these files.

Please refer to: https://github.com/apache/fesod/blob/main/fesod-sheet/src/main/java/org/apache/fesod/sheet/FesodSheet.java

[56wj]

Updated — I removed the EasyExcel-derived header from the new converter file and retained only the ASF license header, following FesodSheet.java. The focused converter/loader tests and Spotless check pass on the updated commit 3b7ae8f.