fix(spec): Include delete file content to V3 manifest

[CTTY]

Which issue does this PR close?

• Closes V3 manifest writer does not write content metadata field #1973

What changes are included in this PR?

• Write content to V3 manifest so the field is preserved correctly for delete entry

Are these changes tested?

Yes

[yshcz]

Might be worth using a match on self.metadata.format_version here similar to the pattern used in avro_schema above, so the compiler can catch any unhandled versions
in the future.

        match self.metadata.format_version {
            FormatVersion::V1 => {}
            FormatVersion::V2 | FormatVersion::V3 => {
                avro_writer
                    .add_user_metadata("content".to_string(), self.metadata.content.to_string())?;
            }
        }