[Feature Request]: --svd-{dir|path} and --z123-{dir|path} launch arguments

### Is there an existing issue for this?

- [X] I have searched the existing issues and checked the recent builds/commits

### What would your feature do ?

Add launch arguments to determine the directory where both svd and z123 checkpoints will be looked for.

### Proposed workflow

1. Go your launch script
2. Add --svd-dir (or --svd-path), --z123-dir (or --z123-path) to the launch arguments.
3. Save your launch script.

### Additional information

These are some already existing launch arguments (from either AUTOMATIC1111 or extensions) that solve this feature for other model types.

--ckpt-dir
--vae-dir
--gfpgan-dir
--codeformer-models-path
--gfpgan-models-path
--esrgan-models-path
--bsrgan-models-path
--realesrgan-models-path
--scunet-models-path
--swinir-models-path
--ldsr-models-path
--lora-dir
--clip-models-path
--embeddings-dir
--textual-inversion-templates-dir
--hypernetwork-dir
--localizations-dir
--controlnet-dir
--lyco-dir-backcompat

Start by locating the launch script and the existing handling for arguments such as --ckpt-dir and --vae-dir. Trace how those arguments select model directories, then determine where the svd and z123 checkpoint lookup is configured. Done means the launch arguments are accepted and each selects the corresponding checkpoint directory.

Written by the indexing model from the issue text.