Update on "[ET-VK] Adding reserve and append functions to SpecVarList"

This diff adds two new functions to the SpecVarList class in the Vulkan runtime library. The first function, reserve, allows the user to reserve a certain amount of space in the SpecVarList before adding any elements. The second function, append, allows the user to add a single SpecVar to the SpecVarList. These functions are useful for optimizing memory usage and improving performance in the Vulkan runtime.

Differential Revision: [D70021782](https://our.internmc.facebook.com/intern/diff/D70021782/)

[ghstack-poisoned]

Author: trivedivivek