[C++][Gandiva] Add string functions in Gandiva

[asfimport]

Add following functions in Gandiva -

substr(str, offset, len) : returns substring of length len starting from offset. negative offset is counted from string end.

substr(str, offset) : returns substring starting from offset. negative offset is counted from string end.

concat(str1, str2) : concatenate two strings

castVARCHAR(timestamp, len) : returns timestamp as string of length len

convert_fromUTF8(binary): convert binary array to string

Reporter: Prudhvi Porandla / @pprudhvi
Assignee: Prudhvi Porandla / @pprudhvi

PRs and other links:

• GitHub Pull Request #4942

_{Note: This issue was originally created as ARROW-6034. Please see the migration documentation for further details.}

[asfimport]

Pindikura Ravindra / @pravindra:
Issue resolved by pull request 4942
#4942