[FEA] Expand string operator support in libcudf ASTs

After adding string scalar support in #13061, libcudf now supports string column and string scalar operands, plus comparison operators.

There are additional operators that would be useful for string types, and this issue will track the most-requested operators.

in scope:
string contains, startswith, endswith
length, find, rfind

out of scope for now:
casting to and from string to other AST types
regex support
concatenation

(Also see background issue #8858)