Rework cudf::io::text::byte_range_info class member functions

[davidwendt]

Description

Adds const declarations to appropriate member functions in class cudf::io::text::byte_range_info and moves the ctor implementation to .cpp file.
This helps with using the byte_range_info objects in const variables and inside of const functions.

Found while working on #15983

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]

/merge