fix: Set Expression serialization to use 'values'

[geruh]

Related to #2775

Rationale for this change

Update the Set Expression serialization to align with the REST spec. Today, the set expression literals are serialized under items, instead of values.

For instance:

{
  "term": "foo",
  "type": "not-in",
  "items": [
    1,
    2,
    3
  ]
}

When it should be:

{
  "term": "foo",
  "type": "not-in",
  "values": [
    1,
    2,
    3
  ]
}

Rest ref: https://github.com/apache/iceberg/blob/47d5f5009eafbbb526e6e2c9cbeac3105bf34670/open-api/rest-catalog-open-api.yaml#L2353

Expression ref: https://github.com/apache/iceberg/blob/d19e3ff07653167d902865281601a5da4e2f2def/core/src/main/java/org/apache/iceberg/expressions/ExpressionParser.java#L189-L192

Are these changes tested?

Yes

Are there any user-facing changes?

only serialization, and initialization stays the same.

[Fokko]

Bumped into the same :D Working on a PR to go from JSON to Expressions

[Fokko]

Thanks @geruh 🙌