Implement optional second argument to ltrim and rtrim functions

[alamb]

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-11164

apache/arrow#9108 adds the ltrim and rtrim functions to datafusion, following postgres. However, postgres offers a second optional argument (the character to trim) that is not yet implemented, as described on https://www.postgresql.org/docs/9.1/functions-string.html

For example:
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This ticket tracks the work to add support for the second optional arguments (what character to trim) - it would probably be a good introductory item for new contributors.

There is more context here:
apache/arrow#9108 (review)

[Jefffrey]

Implemented by bc1e590

Issue can be closed

[Dandandan]

Thanks @Jefffrey