Add relayinfo package and utility modules for NIP-11 support

Introduce the `relayinfo` package with `NIP-11` utilities, including `Fees`, `Limits`, and `NIPs` structures. Add utility modules for handling numbers, timestamps, and kinds. Integrate functionality for fetching and managing relay information.

pkg/utils/number/list.go

@@ -0,0 +1,36 @@
+// Package number implements a simple number list, used with relayinfo package
+// for NIP support lists.
+package number
+
+import "fmt"
+
+// List is a simple list of numbers with a sort implementation and number match.
+type List []int
+
+func (l List) Len() int           { return len(l) }
+func (l List) Less(i, j int) bool { return l[i] < l[j] }
+func (l List) Swap(i, j int)      { l[i], l[j] = l[j], l[i] }
+
+// HasNumber returns true if the list contains a given number
+func (l List) HasNumber(n int) (idx int, has bool) {
+	for idx = range l {
+		if l[idx] == n {
+			has = true
+			return
+		}
+	}
+	return
+}
+
+// String outputs a number.List as a minified JSON array.
+func (l List) String() (s string) {
+	s += "["
+	for i := range l {
+		if i > 0 {
+			s += ","
+		}
+		s += fmt.Sprint(l[i])
+	}
+	s += "]"
+	return
+}