GH-3471: Fix ByteBuffer handling in VariantUtil and VariantBuilder

[rayokota]

Rationale for this change

Fix ByteBuffer handling in VariantUtil and VariantBuilder.

What changes are included in this PR?

• Fix VariantUtil.getMetadataMap reading string bytes from the wrong position
  when the metadata ByteBuffer has a non-zero position(). Changed
  slice(metadata, stringStart + offset) to
  slice(metadata, pos + stringStart + offset) to match the array-backed branch.
• Fix VariantBuilder.appendBinary mutating the caller's ByteBuffer by using
  binary.duplicate(), consistent with appendEncodedValue.

Are these changes tested?

• Added TestVariantObject.testMetadataWithNonZeroPositionReadOnly: constructs a
  metadata ByteBuffer with a non-zero position and read-only flag, then verifies
  ImmutableMetadata (which calls getMetadataMap) correctly parses the
  dictionary. Fails without the fix.
• Added TestVariantScalarBuilder.testBinaryBuilderDoesNotMutateCallerBuffer:
  verifies that appendBinary does not change the caller's ByteBuffer position
  or remaining count.

Are there any user-facing changes?

• No

Closes #3471