Add move_to function to cudf::string_view iterator

[davidwendt]

Description

Adds a move_to() function the cudf::string_view::const_iterator class to help minimize character counting when creating and incrementing the iterator on multi-byte UTF8 characters.
The function simply moves the iterator from the current character position to the given one. This is just a shortcut for the form

itr += (new_position - itr.position());

This pattern is repeated many times in #13322 and likely future PRs that require the same behavior.
The PR also includes an update to the string_view::begin() to set the byte-offset directly rather than waste instructions calculating it.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[davidwendt]

Messed up moving this to 23.08 so closing this one and re-opening as #13428