Support TGI as another alternative backend

[kerthcet]

What would you like to be added:

TGI is also a popular inference backend that we should support. More importantly, we should set this as an example once people would like to support another backend in the future.

Why is this needed:

Adopte more backends and make this a reference.

Completion requirements:

This enhancement requires the following artifacts:

• Design doc
• API change
• Docs update

The artifacts should be linked in subsequent comments.

[kerthcet]

/kind feature

[kerthcet]

/milestone v0.1.0

[kerthcet]

A follow up of #134

[kerthcet]

/help

[kerthcet]

AFAIK, what we need to do now is:

• Added a new in-tree backend in api/inference/v1alpha1/config_types.go
• Add a default backendRuntime for TGI in chart/templates/backends/
• Add a webhook test to validate that text-generation-inference backendRuntime is supported by default, under the file: /test/integration/webhook/playground_test.go`

We need e2e tests for this, however, we're lack of GPUs right now, so if TGI supports CPU inference, we can add an e2e test for it, just like we do for llama.cpp, otherwise, let's leave it for a follow up until we tackle the GPU shortage.