Enable ripgrep to run on symbolic links when using +vertico-file-search

### Describe your request

Currently `+vertical-file-search` function does not allow any customization of the search argument for ripgrep, as in the source code:
```
(consult-ripgrep-args
(concat "rg "
(if all-files "-uu ")
(unless recursive "--maxdepth 1 ")
"--null --line-buffered --color=never --max-columns=1000 "
"--path-separator / --smart-case --no-heading --line-number "
"--hidden -g !.git -g !.svn -g !.hg "
(mapconcat #'shell-quote-argument args " ")
" ."))
```
where the `consult-ripgrep-args` is always overwrited. currently there is no way to add option `-L` for searching in symbolic link.
I wonder if it will make sense to add a global variable like `rg-extra-args` where we can do something like:
```
(consult-ripgrep-args
(concat "rg "
(if all-files "-uu ")
(unless recursive "--maxdepth 1 ")
"--null --line-buffered --color=never --max-columns=1000 "
"--path-separator / --smart-case --no-heading --line-number "
"--hidden -g !.git -g !.svn -g !.hg "
(mapconcat #'shell-quote-argument args " ")
rg-extra-args
" ."))
```

### Briefly explain its use-case

1. default value of `rg-extra-args` is empty string `""`
2. configure `rg-extra-args` to have something like `-L` for symbolic link or any additional args for rg command
3. now you have things enabled as needed