Allow custom prefix/suffix functions to be defined in build endpoint

[Vealor]

See branch custom_functions

[Vealor]

@JoshuaPortelance can it be possible to use multiple prefix/suffix functions? Can you use multiple activator too?
It seems that this

neurx/lib/neurx/endpoints/build.ex

Lines 166 to 177 in 352615a

	defp sanitize_function_list(funcs) do
	if funcs != nil do
	funcs |>
	Enum.map(fn f ->
	if f != nil do
	f
	end
	end)
	else
	[]
	end
	end

only allows for a single one to be used as it returns the function, else it returns a list. Will likely have to be modified to push onto a new list and return that list if we are considering multiple functions. If there is only one, then no list is required. Take a look at the example in the function comment.

[Vealor]

Nevermind, it just removes anything that's not nil.

[Vealor]

Putting prefix/suffix on the backburner for later, not required right now.