Remove the boost/algorithm/string/case_conv.hpp dependency

Summary:

• Implements ParseNetwork unit test.

This commit implements a unit test that validates the ParseNetwork(std::string) implementation in netbase.cpp.

• Implements custom tolower and toupper functions.

This commit implements custom equivalents for the C and C++ tolower and toupper Standard Library functions.
In addition it implements a utility function to capitalize the first letter of a string.

• Removes the Boost case_conv.hpp dependency.

This commit removes the boost/algorithm/string/case_conv.hpp dependency from the project. It replaces the boost::to_lower and boost::to_upper functions with custom functions that are locale independent and ASCII deterministic.

This is a backport of Core PR13671

Depends on D2824

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D2825