chore: Split values.rs into separate files

[mbutrovich]

Which issue does this PR close?

• Closes Split iceberg/src/spec/values.rs into multiple files #1807.

What changes are included in this PR?

Split up values.rs into component files in a values folder. Happy to discuss organization. Here are the details to account for line differences:

1. Copyright headers duplicated - Each of the new files got its own Apache license header
2. Module structure added - Created mod.rs with module declarations and re-exports
3. Module-level docs added - Changed from /*! Value in iceberg */ to individual //! style doc comments for each
   module
4. Visibility changes - Changed 5 constants from private const to pub(crate) const. This was necessary so these constants could be accessed across the split files:
   • MAX_TIME_VALUE
   • INT_MAX
   • INT_MIN
   • LONG_MAX
   • LONG_MIN

Are these changes tested?

Existing tests.

[liurenjie1024]

Thanks @mbutrovich for this fix! I didn't expect it has grown to so large.