add sql function support for deep filters

[bramca]

This PR is a addition for the last step of adding nested filters.
Supporting SQL functions for in the deepest level of the nested filter. e.g.:

filters := []map[string]any{
	{
		"LOWER(occupation)": []string{"~op", "~de"},
	},
	{
		"nested": map[string]any{
			"LOWER(name)": "katherina",
		},
	},
	{
		"nested": map[string]any{
			"UPPER(occupation)": []string{"DEV", "OPS"},
		},
	},
}

I wanted this as an extension of my odata filtering functionality, so that it also supports filter queries like:

GET http://localhost:8080/api/v1/something?$filter=contains(tolower(nested/value),'value')

Have a branch open for this on my odata repo, for now with a fork of this repo.