feat: support custom selection dialog

[ductridev]

This PR adds support for a custom selection dialog via a new custom builder. An example usage of the custom builder is also provided.

Changes Made:

• Implemented custom logic for showCountryCodePickerDialog (commit 0dd5c35)
• Created an example demonstrating how to use the custom builder (commit 70851f6)

Related Issues:
#52

Note:

• This PR is only implemented to support custom showCountryCodePickerDialog while showCountryCodePickerBottomSheet is not implemented yet
• This is my first PR on this repo—any feedback is appreciated!

cc: @chandrabezzo @MrRoy121

[chandrabezzo]

Hi @ductridev can you provide the proof of your changes?