fn: add Collect+CollertErr function for iterators

In this commit, we introduce a new utility function `Collect` to the fn
package. This function drains all elements from an iterator and returns
them as a slice. This is particularly useful when transitioning from
iterator-based APIs to code that expects slices, allowing for gradual
migration to the new iterator patterns.

The fn module's go.mod is also updated to require Go 1.23, which is
necessary for the built-in iter.Seq type support.

The replace directive will be removed once the fn package changes are
merged and a new version is tagged.

fn/go.mod

@@ -1,6 +1,6 @@
 module github.com/lightningnetwork/lnd/fn/v2
 
-go 1.19
+go 1.23
 
 require (
 	github.com/stretchr/testify v1.8.1