[Feature]: Enable setting `create_mr.title` in as function in `setup` options

Thanks for `gitlab.nvim`! Just started using this plugin today and I think it will be incredibly useful for my workflow.

## Feature Description

After referencing [`:h gitlab.nvim`](https://github.com/Drew-Daniels/gitlab.nvim/blob/e29909cd1064a7b53c3150bff49449a548dadf8d/doc/gitlab.nvim.txt#L866), I can see that we can set the title for Merge Requests using `require("gitlab").create_mr({ title = "Fix bug XYZ", description = "Closes #123" })`, but we cannot set MR titles in the `setup` options.

This makes sense, because it would rarely be useful to always use the same title for every Merge Request, but I think it would be a useful option if not only users were able to also set `settings.create_mr.title` within our `setup` configuration to `gitlab.nvim`, but we were able to set the title by providing a function that will be called to generate the title - so this is sort of 2 feature requests.

1. To allow users to configure `settings.create_mr.title` within the `setup` options and
2. To allow users to provide a `lua` function to this property, that, when called will return a string that can be used for the GitLab MR.

## Ideal Behavior

```lua
return {
"harrisoncramer/gitlab.nvim",
dependencies = {
"MunifTanjim/nui.nvim",
"nvim-lua/plenary.nvim",
"sindrets/diffview.nvim",
"stevearc/dressing.nvim",
"nvim-tree/nvim-web-devicons",
},
build = function()
require("gitlab.server").build(true)
end, -- Builds the Go binary
config = function()
require("gitlab").setup({
create_mr = {
target = "main",
template_file = "Default.md",
title = function()
-- My complicated function that will use some runtime logic to return a string to use for MR title
return "foo(bar): baz"
end,
},
})
end,
}
```

Thanks again!

Inizia dalla documentazione di configurazione in `doc/gitlab.nvim.txt` e segui gli entry point `require(\"gitlab\").setup` e `create_mr` descritti nell'issue. Il lavoro è completato quando `settings.create_mr.title` accetta una stringa o una funzione Lua e la stringa risultante viene usata per il titolo del GitLab Merge Request.

Scritto dal modello di indicizzazione a partire dal testo della issue.