[SPARK-28123][SQL] String Functions: support btrim

[beliefer]

What changes were proposed in this pull request?

Spark support trim/ltrim/rtrim now. The function btrim is an alternate form of TRIM(BOTH <chars> FROM <expr>).
btrim removes the longest string consisting only of specified characters from the start and end of a string.

The mainstream database support this feature show below:

Postgresql
https://www.postgresql.org/docs/11/functions-binarystring.html

Vertica
https://www.vertica.com/docs/9.2.x/HTML/Content/Authoring/SQLReferenceManual/Functions/String/BTRIM.htm?tocpath=SQL%20Reference%20Manual%7CSQL%20Functions%7CString%20Functions%7C_____5

Redshift
https://docs.aws.amazon.com/redshift/latest/dg/r_BTRIM.html

Druid
https://druid.apache.org/docs/latest/querying/sql.html#string-functions

Greenplum
http://docs.greenplum.org/6-8/ref_guide/function-summary.html

Why are the changes needed?

btrim is very useful.

Does this PR introduce any user-facing change?

Yes. btrim is a new function

How was this patch tested?

Jenkins test.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/38509/

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/38509/

[SparkQA]

Test build #133922 has finished for PR 31128 at commit 736027e.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/38521/

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/38521/

[SparkQA]

Test build #133933 has finished for PR 31128 at commit e4814dd.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[beliefer]

cc @cloud-fan

[srowen]

There are a number of PRs floating around about this. For the record, I don't strongly oppose it, but, does not seem like a semantically useful function, even if some legacy DBs support it.

[beliefer]

There are a number of PRs floating around about this. For the record, I don't strongly oppose it, but, does not seem like a semantically useful function, even if some legacy DBs support it.

This PR is different from the others, this is also because my previous misunderstanding of btrim. At the first, I Investigated the PostgreSql, the implement of btrim in PostgreSql accepts byte array and return byte array as result.
So I mistakenly thought that btrim's behavior is like this.

Then I created two PR as you know. But the two PR try to let btrim or trim accepts byte array and return byte array as result.

After that, I Investigated other database and found btrim means trim from both ends of source string and it is just an alternate form of TRIM(BOTH <chars> FROM <expr>).
btrim is a little similar to ltrim or rtrim.

This PR only need minor changes (such as giving trim an alias) can support it.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/38579/

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/38579/

[cloud-fan]

it's a bit bad to drop these examples.

[cloud-fan]

Maybe we should think about a better way to add function alias, so that they can have their own examples.

[SparkQA]

Test build #133991 has finished for PR 31128 at commit e9b95e8.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[beliefer]

The function adopt the new PR #31390