feat(bigquery)!: Add support for STRING function

[VaggelisD]

This PR adds support for BQ's STRING(...) function, which turns input expressions into STRINGs. There are two supported versions:

1. JSON based: STRING(json_expr)
2. TIMESTAMP based: STRING(timestamp_expression[, time_zone])

Both variants are transpiled to CAST(... AS VARCHAR) for all other dialects; For the (2) variant with time_zone supplied, the timestamp is passed through exp.ConvertTimezone for the timezone conversion:

bigquery> SELECT STRING("2008-12-25 15:30:00", "America/New_York") AS string;
2008-12-25 10:30:00-05

snowflake> SELECT CAST(CONVERT_TIMEZONE('UTC', 'America/New_York', '2008-12-25 15:30:00') AS VARCHAR);
2008-12-25 10:30:00.000

duckdb> SELECT CAST(CAST('2008-12-25 15:30:00' AS TIMESTAMP) AT TIME ZONE 'UTC' AT TIME ZONE 'America/New_York' AS TEXT) AS color;
┌─────────────────────┐
│        color        │
│       varchar       │
├─────────────────────┤
│ 2008-12-25 10:30:00 │
└─────────────────────┘

Docs

BQ JSON STRING() | BQ TIMESTAMP STRING()