les, les/lespay/client: add service value statistics and API (#20837)

This PR adds service value measurement statistics to the light client. It
also adds a private API that makes these statistics accessible. A follow-up
PR will add the new server pool which uses these statistics to select
servers with good performance.

This document describes the function of the new components:
https://gist.github.com/zsfelfoldi/3c7ace895234b7b345ab4f71dab102d4

Co-authored-by: rjl493456442 <garyrong0905@gmail.com>
Co-authored-by: rjl493456442 <garyrong0905@gmail.com>

internal/web3ext/web3ext.go

@@ -33,6 +33,7 @@ var Modules = map[string]string{
 	"swarmfs":    SwarmfsJs,
 	"txpool":     TxpoolJs,
 	"les":        LESJs,
+	"lespay":     LESPayJs,
 }
 
 const ChequebookJs = `
@@ -856,3 +857,34 @@ web3._extend({
 	]
 });
 `
+
+const LESPayJs = `
+web3._extend({
+	property: 'lespay',
+	methods:
+	[
+		new web3._extend.Method({
+			name: 'distribution',
+			call: 'lespay_distribution',
+			params: 2
+		}),
+		new web3._extend.Method({
+			name: 'timeout',
+			call: 'lespay_timeout',
+			params: 2
+		}),
+		new web3._extend.Method({
+			name: 'value',
+			call: 'lespay_value',
+			params: 2
+		}),
+	],
+	properties:
+	[
+		new web3._extend.Property({
+			name: 'requestStats',
+			getter: 'lespay_requestStats'
+		}),
+	]
+});
+`