Add support for week in extract operator

[Ted-Jiang]

close #435

[liukun4515]

LGTM

[Ted-Jiang]

Test with datafusion!

un  --bin datafusion-cli
   Compiling sqlparser v0.15.0 (/Users/yangjiang/CLionProjects/sqlparser-rs)
   Compiling datafusion-common v7.0.0 (/Users/yangjiang/CLionProjects/arrow-datafusion/datafusion-common)
   Compiling datafusion-expr v7.0.0 (/Users/yangjiang/CLionProjects/arrow-datafusion/datafusion-expr)
   Compiling datafusion-physical-expr v7.0.0 (/Users/yangjiang/CLionProjects/arrow-datafusion/datafusion-physical-expr)
   Compiling datafusion v7.0.0 (/Users/yangjiang/CLionProjects/arrow-datafusion/datafusion)
   Compiling datafusion-cli v7.0.0 (/Users/yangjiang/CLionProjects/arrow-datafusion/datafusion-cli)
    Finished dev [unoptimized + debuginfo] target(s) in 42.48s
     Running `target/debug/datafusion-cli`

DataFusion CLI v7.0.0
❯ select EXTRACT(WEEK FROM to_timestamp('2020-09-08T12:00:00+00:00'));
+-----------------------------------------------------------------------+
| datepart(Utf8("WEEK"),totimestamp(Utf8("2020-09-08T12:00:00+00:00"))) |
+-----------------------------------------------------------------------+
| 37                                                                    |
+-----------------------------------------------------------------------+
1 row in set. Query took 0.005 seconds.
❯

[Ted-Jiang]

@alamb PTAL

[coveralls]

Pull Request Test Coverage Report for Build 1966663023

• 2 of 3 (66.67%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.009%) to 90.44%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	1	2	50.0%

Totals
Change from base Build 1953203878:	-0.009%
Covered Lines:	7275
Relevant Lines:	8044

---

💛 - Coveralls

[alamb]

Thanks @Ted-Jiang @liukun4515 and @houqp