Add option to invoke callout prior to each instruction

[Alan-Jowett]

This pull request introduces a new debugging feature for the ubpf virtual machine, allowing a debug function to be registered and invoked before each instruction execution. The most important changes include adding the debug function registration and invocation, updating the VM state structure, and creating a new test to verify this functionality.

Debugging Feature Enhancements:

• Added a new typedef ubpf_debug_fn and the function ubpf_register_debug_fn to ubpf.h to allow the registration of a debug function that gets invoked before each instruction. (vm/inc/ubpf.h)
• Updated the ubpf_vm structure to include the debug function and its context. (vm/ubpf_int.h)
• Modified ubpf_exec_ex to call the registered debug function before executing each instruction. (vm/ubpf_vm.c)
• Implemented the ubpf_register_debug_fn function to handle the registration and validation of the debug function. (vm/ubpf_vm.c)

Testing and Documentation:

• Created a new test source file ubpf_test_debug_function.cc to verify that the debug function is correctly registered and invoked, capturing the VM state before each instruction. (custom_tests/srcs/ubpf_test_debug_function.cc)
• Added a test description file ubpf_test_debug_function.md to document the purpose and functionality of the new test. (custom_tests/descrs/ubpf_test_debug_function.md)
• Included a test input file ubpf_test_debug_function.input to provide the necessary data for the new test. (custom_tests/data/ubpf_test_debug_function.input)

[coveralls]

coverage: 79.637%. first build
when pulling 1640801 on Alan-Jowett:single_step
into f81cba0 on iovisor:main.