chore: Better randomness (#6755)

## About the changes
This change is irrelevant as it doesn't pose a security risk, but
there's no reason for us not to use a different type of random
generation for the `sessionId`

**Note:** the magic number 18 was picked because that's the length of
the string we get from `String(Math.random())`

Closes https://github.com/Unleash/unleash/security/code-scanning/68 and
https://github.com/Unleash/unleash/security/code-scanning/69

src/lib/features/frontend-api/create-context.ts

@@ -1,9 +1,7 @@
 // Copy of https://github.com/Unleash/unleash-proxy/blob/main/src/create-context.ts.
-
-/* eslint-disable prefer-object-spread */
+import crypto from 'crypto';
 import type { Context } from 'unleash-client';
 
-// eslint-disable-next-line @typescript-eslint/explicit-module-boundary-types
 export function createContext(value: any): Context {
     const {
         appName,
@@ -33,9 +31,8 @@ export function createContext(value: any): Context {
     return cleanContext;
 }
 
-// eslint-disable-next-line @typescript-eslint/explicit-module-boundary-types
 export const enrichContextWithIp = (query: any, ip: string): Context => {
     query.remoteAddress = query.remoteAddress || ip;
-    query.sessionId = query.sessionId || String(Math.random());
+    query.sessionId = query.sessionId || crypto.randomBytes(18).toString('hex');
     return createContext(query);
 };

src/lib/features/frontend-api/frontend-api-service.ts

@@ -1,3 +1,4 @@
+import crypto from 'crypto';
 import type {
     IUnleashConfig,
     IUnleashServices,
@@ -88,7 +89,8 @@ export class FrontendApiService {
     ): Promise<FrontendApiFeatureSchema[]> {
         const client = await this.clientForFrontendApiToken(token);
         const definitions = client.getFeatureToggleDefinitions() || [];
-        const sessionId = context.sessionId || String(Math.random());
+        const sessionId =
+            context.sessionId || crypto.randomBytes(18).toString('hex');
 
         const resultDefinitions = definitions
             .filter((feature) =>
@@ -115,7 +117,8 @@ export class FrontendApiService {
     ): Promise<FrontendApiFeatureSchema[]> {
         const client = await this.newClientForFrontendApiToken(token);
         const definitions = client.getFeatureToggleDefinitions() || [];
-        const sessionId = context.sessionId || String(Math.random());
+        const sessionId =
+            context.sessionId || crypto.randomBytes(18).toString('hex');
 
         const resultDefinitions = definitions
             .filter((feature) => {