assume by default microsecond precision for timestamp

to align with spark https://spark.apache.org/docs/3.4.2/api/java/org/apache/spark/sql/types/TimestampType.html

until we have proper PrecisionTimestamp substrait-io/substrait#659

datafusion/substrait/src/variation_const.rs

@@ -35,9 +35,9 @@
 /// The "system-preferred" variation (i.e., no variation).
 pub const DEFAULT_TYPE_VARIATION_REF: u32 = 0;
 pub const UNSIGNED_INTEGER_TYPE_VARIATION_REF: u32 = 1;
-pub const TIMESTAMP_SECOND_TYPE_VARIATION_REF: u32 = 0;
+pub const TIMESTAMP_SECOND_TYPE_VARIATION_REF: u32 = 2;
 pub const TIMESTAMP_MILLI_TYPE_VARIATION_REF: u32 = 1;
-pub const TIMESTAMP_MICRO_TYPE_VARIATION_REF: u32 = 2;
+pub const TIMESTAMP_MICRO_TYPE_VARIATION_REF: u32 = 0;
 pub const TIMESTAMP_NANO_TYPE_VARIATION_REF: u32 = 3;
 pub const DATE_32_TYPE_VARIATION_REF: u32 = 0;
 pub const DATE_64_TYPE_VARIATION_REF: u32 = 1;