feat: Improve display of Decimal values

[neilconway]

Which issue does this PR close?

• Closes Improve Display of Decimal values #22485.

Rationale for this change

Displaying Decimal values (e.g., in EXPLAIN output or in auto-generated column names) included Rust's Some notation. For example:

> set datafusion.sql_parser.parse_float_as_decimal = true; explain select 0.1;

Eliding the rest of the EXPLAIN output, the projection is rendered as as Decimal128(Some(1),1,1): Some(1),1,1.

This is not very readable:

1. Users may not understand the Rust Option syntax
2. When the value is None, we should print NULL, so showing the Option is not useful anyway
3. The user needs to do math to convert the stored value (1) into the actual value (0.1)

This PR changes how Decimal values are displayed. For example, the query above would show the projection as: Decimal128(0.1,1,1): 0.1.

What changes are included in this PR?

• Update fmt::Display and fmt::Debug for ScalarValue to use the new format for Decimal values
• Rename fmt_binary to fmt_binary_debug for consistency (this format is used by Debug, not Display)
• Add unit tests for new formatting behavior
• Update SLTs / goldens for new output format
• Update migration guide for new output format

Are these changes tested?

Yes; new tests added and expected test output has been updated. The implemented behavior was compared with Postgres and DuckDB, and roughly matches what those other systems do.

Are there any user-facing changes?

Yes. This change may break users that have EXPLAIN output or auto-generated column names in golden files, and other places where the Debug / Display of a ScalarValue is being compared against. A section has been added to the 55.0.0 migration guide.