added base code for gnmi api client

[Meghna Mishra (meghna-mishra)]

What I did

Added base code for GnmiApiClient

1. Accepts SHOW CLI command as string (base implemented, short options not implemented yet).
2. Connects to telemetry and gets JSON result from SHOW gNMI (not implemented yet, returns default output).
3. Converts JSON result to CLI tabular output (not implemented yet, returns default output, need to port implemented code from a different source later).

How I did it

Modified the following file.
setup.py: added 'gnmi_api_client' to the packages list

Added the following files.
client.py: GnmiApiClient orchestrator + CLI main() (test-mode only)
input_formatter.py: ShowCliToGnmiPathConverter — CLI tokens → gNMI path string
output_formatter.py: DummyTabularFormatter — JSON → table (test-mode only)
__init__.py: re-exports GnmiApiClient, main

How to verify it

Verified the running of main() and input formatter on BJLab device.

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

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