feat: Expose both object and method classes for builtin commands

[mkundu1]

Supports following syntaxes for built-in commands like read-case:

import ansys.fluent.core as pyfluent

read_case_obj = pyfluent.solver.ReadCase(settings_source = <solver_session>)  # returns the command object
read_case_obj.is_active()
read_case(file_name=<case_file>)

pyfluent.solver.read_case(settings_source = <solver_session>, file_name=<case_file>)  # executes the command

[hpohekar]

Looks good to me.