Add DCT implementation

## Description

Add DCT implementation, plus tests and benchmarks, for transforms of size with no factors other than 2, 3, 5 and 7 (all others currently return unsupported).

## Checklist

* \[x\] [Contribution meets RAL's licence terms](https://gitlab.arm.com/networking/ral/-/blob/main/CONTRIBUTING.md#user-content-licensing-information)
* \[x\] [New functions adhere to RAL's naming scheme](https://gitlab.arm.com/networking/ral/-/blob/main/CONTRIBUTING.md#user-content-function-naming)
* \[x\] [Contribution conforms to RAL's directory structure](https://gitlab.arm.com/networking/ral/-/blob/main/CONTRIBUTING.md#user-content-directory-structure)
* \[x\] [Documentation updated](https://gitlab.arm.com/networking/ral/-/blob/main/CONTRIBUTING.md#user-content-documentation)
* \[\] ["Unreleased" section of the Changelog updated](https://gitlab.arm.com/networking/ral/-/blob/main/CHANGELOG.md#unreleased) - I will do this in a follow-on patch 
* \[x\] [`clang-format` and `clang-tidy` run and changes included (C/C++ code)](https://gitlab.arm.com/networking/ral/-/blob/main/CONTRIBUTING.md#user-content-cc-code-style)
* \[x\] [`flake8` run and changes included (Python code)](https://gitlab.arm.com/networking/ral/-/blob/main/CONTRIBUTING.md#user-content-python-code-style)
* \[x\] [`cmake-format` run and changes included (CMake code)](https://gitlab.arm.com/networking/ral/-/blob/main/CONTRIBUTING.md#user-content-cmake-code-style)
* \[x\] [Tests added or updated](https://gitlab.arm.com/networking/ral/-/blob/main/CONTRIBUTING.md#user-content-writing-tests)
* \[x\] [Tests pass when run with AddressSanitizer](https://gitlab.arm.com/networking/ral/-/blob/main/CONTRIBUTING.md?ref_type=heads#user-content-testing-with-addresssanitizer)
* \[x\] [Benchmarks added or updated](https://gitlab.arm.com/networking/ral/-/blob/main/CONTRIBUTING.md#user-content-writing-benchmarks)