[Mikkey-f]

Purpose of the pull request

Related: #1017

Adds a built-in converter for java.time.OffsetTime, the last JDK 8 java.time type without one. On current main a POJO field of this type fails on read and write with Can not find 'Converter' support class OffsetTime.

Scope follows the proposal posted in #1017 (string only, no objections raised in #1017): Excel NUMBER/DATE cells store a wall-clock time without an offset, so a numeric path would have to fabricate one.

What's changed?

• New converters/offsettime/OffsetTimeStringConverter, mirroring OffsetDateTimeStringConverter: default format ISO_OFFSET_TIME, @DateTimeFormat patterns honored, locale-aware through DateUtils.
• New DateUtils.parseOffsetTime(...) / DateUtils.format(OffsetTime, ...) helpers, same shape as the existing OffsetDateTime ones. Empty/null format falls back to ISO_OFFSET_TIME; text without an offset is rejected rather than defaulted.
• Registered with putAllConverter (read), putWriteConverter (XLSX/XLS write) and putWriteStringConverter (CSV). The proposal mentioned the first and third; putWriteConverter is also needed because a non-CSV write resolves the converter key without a cell type — the same registration used by UUIDStringConverter and InstantStringConverter.
• Tests: converter unit tests (keys, ISO/custom format, empty/null pattern, locale, missing/invalid offset, registration), DateUtils helper tests, and an XLSX/XLS/CSV round trip covering both the default ISO format and a @DateTimeFormat pattern.

Verification

• ./mvnw spotless:check passes.
• ./mvnw clean package -Dmaven.test.skip=false -pl fesod-common,fesod-shaded,fesod-sheet: 1,895 tests, 0 failures.
• ./mvnw javadoc:javadoc passes.
• Red side: without the registrations the three round-trip cases fail with Can not find 'Converter' support class OffsetTime, and the registration test fails on a null converter.

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.