[Feature]: Add possibility to override plugin loading event

[epontan]

Description

It can be useful be able to override which event a core plugin should be loaded from the user configuration.

Add an event attribute to the default configuration of all core plugins that have an event configured and refer to that from the plugins module.

This implements the approach suggested in #1692.

How Has This Been Tested?

My use case was to make gitsigns.nvim load on startup when not starting vim with a file as argument (not only when BufRead event is triggered).
I use the gitsigns plugin to review all the changes of last commit by changing its base (e.g. HEAD~1).

In my config.lua I override the default configuration like this:

lvim.builtin.gitsigns.event = "VimEnter"

This makes the plugin available at start without having to open a file first.

[kylo252]

I've been thinking about this for a while for the commit part which has now become relevant, so we still need to handle the rest of the supported attributes. I'm not sure what is a clean way to do this, so maybe we should instead resurrect #1692, or something similar?