feat!: use vim.ui.select for AvanteSwitchProvider

[teto]

vim.ui.input is less practical then vim.ui.select which shows all models without having to trigger completion.
Also vim.ui.select can be overriden by pickers like snack or fzf-lua (https://github.com/ibhagwan/fzf-lua#neovim-api) eg:

Now this can be seen as a breaking change because:

• AvanteSwitchProvider MODEL is not accepted anymore

In my first attempt I had AvanteSwitchProvider PREFIX filter the models before showing them to the user.

If you agree with my change, let me know how you want to deal with the "breaking" behavior ?

1. we change nothing. A user updates its config to :lua require("avante.api").switch_provider("mistral") instead ?
2. AvanteSwitchProvider MODEL does switch provider bypassing vim.ui.select (and/or MODEL acts as a filter)
3. we keep AvanteSwitchProvider and introduce AvanteSelectProvider instead ? since it's annoying to keep both we deprecate AvanteSwitchProvider ?

Worst case I can keep this change in my own init.lua but as I found this version more helpful than avante's default I prefer to upstream.