chore: Expose TRITONSERVER_Server instance in Python In-Process API

[KrishnanPrash]

Goal: To be able to start frontends in python, given that the server instance is being created with the Python In-Process API (tritonserver).

By exposing the c++ pointer to a running TRITONSERVER_Server instance, we can retrieve the memory address of the instance in python and pass it to the tritonfrontend python package. This package will then use reinterepet_cast<...> and recover the TRITONSERVER_Server object from the memory address given.

[nnshah1]

@KrishnanPrash , @GuanLuo , @rmccorm4 -

Two points:

1. Instead of passing the underlying TRITONSERVER object between modules - could we pass the PyServer object? Reasoning: Can avoid directly exposing the TRITONSERVER ptr and leverage PyBind / ctypes to access the pointer to PyServer. If this is not possible / advisable due to ABI compat - would just like confirmation to understand.

2. If we need to expose the ptr, let's make it private with a leading underscore and also remove the get_c - as we don't use get / set generally.

[KrishnanPrash]

@nnshah1

1. To my understanding, passing the PyServer object would not be sufficient, because tritonfrontend (and by extension server repo) does not have access to the PyServer implementation and therefore cannot recover the underlying pointer to the TRITONSERVER_SERVER object.
2. Thank you for the suggestion, the new function name should be just _ptr().