[ET-VK] Adding reserve and append functions to SpecVarList (#8670)

* [ET-VK] Adding a workgroup class to VecUtils

Pull Request resolved: https://github.com/pytorch/executorch/pull/8632

This diff adds a new class called `WorkgroupSize` to the `VecUtils` header file. The `WorkgroupSize` class takes three `uint32_t` values as parameters and stores them in a single `uint32_t` variable using bitwise operations. This class is used in the Vulkan backend to specify the size of a workgroup for a given operation.
ghstack-source-id: 268172661
@exported-using-ghexport

Differential Revision: [D70021019](https://our.internmc.facebook.com/intern/diff/D70021019/)

* [ET-VK] Adding reserve and append functions to SpecVarList

Pull Request resolved: https://github.com/pytorch/executorch/pull/8633

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.
ghstack-source-id: 268172659
@exported-using-ghexport

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

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Co-authored-by: Vivek Trivedi <5340687+trivedivivek@users.noreply.github.com>