[WEB-3787] fix: project joining date (#7127)

* fix: return project joining date

* fix: added project's joining date

* fix: set created_at as read_only_fields

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Co-authored-by: gakshita <akshitagoyal1516@gmail.com>

web/core/store/member/project-member.store.ts

@@ -127,7 +127,10 @@ export class ProjectMemberStore implements IProjectMemberStore {
     const memberDetails: IProjectMemberDetails = {
       id: projectMember.id,
       role: projectMember.role,
-      member: this.memberRoot?.memberMap?.[projectMember.member],
+      member: {
+        ...this.memberRoot?.memberMap?.[projectMember.member],
+        joining_date: projectMember.created_at,
+      },
     };
     return memberDetails;
   });