Add backend capabilities to chat and generate - #1420

This commit introduces the ability to configure and automatically detect
the backend for serving models in InstructLab. One new field, `backend`
has been added to the configuration and the CLI to allow users to
specify the backend. This field is optional.

Additionally, the commit includes the implementation of backend validation and
auto-detection based on the model file properties, specifically targeting GGUF
file format for now. The `serve` command has been updated to utilize these new
configurations and perform backend validation or auto-detection as required.

Unit tests for backend validation and changes to the configuration tests to
include the new fields are also part of this commit.

Co-authored-by: Ali Maredia <amaredia@redhat.com>
Signed-off-by: Sébastien Han <seb@redhat.com>

This commit refactors the backend selection and server initialization
process for both the generate and chat functionalities within the
application. The changes include:

- Abstracting the backend selection logic by utilizing the `backends`
  module, which dynamically determines the appropriate backend based on
  the configuration file or auto-detection if no configuration is set in
  the serve section of config.yaml (serve.backend)

- Encapsulating the conditional logic for checking if the selected
  backend is LLAMA

- Streamlining the exception handling for backend determination and
  server initialization, ensuring that any failures in these processes
  are clearly communicated to the user and result in a graceful exit.

These modifications enhance the code's structure, making it easier to
understand and extend in the future, especially as new backends or
initialization requirements are introduced.

Signed-off-by: Sébastien Han <seb@redhat.com>