feat(spark, bigquery): Add support for UNIX_SECONDS(...)

[VaggelisD]

BigQuery and Spark3/Databricks support UNIX_SECONDS(expr) which returns the number of seconds since 1970-01-01 00:00:00 UTC:

bigquery> SELECT UNIX_SECONDS(TIMESTAMP "2008-12-25 15:30:00+00") AS seconds;
seconds
1230219000

spark-sql (default)> SELECT UNIX_SECONDS(TIMESTAMP "2008-12-25 15:30:00+00") AS seconds;
seconds
1230219000

This PR adds exp.UnixSeconds to enable seamless parsing as well as transpiles the newly added node to DuckDB & Snowflake by utilizing exp.TimestampDiff at a SECOND granularity:

duckdb> SELECT DATE_DIFF('SECONDS', CAST('1970-01-01 00:00:00+00' AS TIMESTAMPTZ), '2008-12-25 15:30:00+00') AS seconds;
┌────────────┐
│  seconds   │
│   int64    │
├────────────┤
│ 1230219000 │
└────────────┘

snowflake> SELECT TIMESTAMPDIFF(SECONDS, CAST('1970-01-01 00:00:00+00' AS TIMESTAMPTZ), '2008-12-25 15:30:00+00') AS seconds;
1230219000

Docs

BQ | Spark3 | Databricks