examples: ExpressionStringify prints day-time intervals under the IntervalYearLiteral label

[nielspardon]

ExpressionStringify.visit(IntervalDayLiteral, ...) (examples/substrait-spark/src/main/java/io/substrait/examples/util/ExpressionStringify.java:133) prints its output under the IntervalYearLiteral label — a copy-paste from the method directly above it:

public String visit(IntervalYearLiteral expr, EmptyVisitationContext context) {
  return "<IntervalYearLiteral " + expr.months() + " " + expr.years() + ">";
}

public String visit(IntervalDayLiteral expr, EmptyVisitationContext context) {
  return "<IntervalYearLiteral " + expr.seconds() + " " + expr.days() + ">";
}

So a plan carrying both interval kinds dumps two entries that both say IntervalYearLiteral, and nothing distinguishes them.

Compounding it, both methods print their components in reverse declaration order with no field labels — months before years, seconds before days — as does visit(IntervalCompoundLiteral, ...) for all five of its fields. <IntervalYearLiteral 1 12> could be 1 year 12 months, 12 years 1 month, or a day-time interval of 1 second and 12 days.

ExpressionStringify is one of the direct ExpressionVisitor implementors, so it is the tool a maintainer reaches for when chasing exactly the kind of interval mis-conversion #1138 describes.

Fix

Correct the label, and label the components while there:

return "<IntervalDayLiteral days=" + expr.days() + " seconds=" + expr.seconds() + ">";

with the same treatment for IntervalYearLiteral and IntervalCompoundLiteral.

Found while reviewing #1140.