Smarter memory protection with Buffer class

Introduces a dedicated `Buffer` abstraction for managing blocks of memory. Buffer comes with protection mechanisms that can be enabled by setting the `KAI_TEST_BUFFER_POLICY` environment variable.

Example usage:

```KAI_TEST_BUFFER_POLICY=PROTECT_OVERFLOW ./kleidiai_test```

Available memory protection mechanisms:

- `KAI_TEST_BUFFER_POLICY=PROTECT_UNDERFLOW`
- `KAI_TEST_BUFFER_POLICY=PROTECT_OVERFLOW`

If `KAI_TEST_BUFFER_POLICY` is not set or is not one of the above values, then no memory protection mechanisms are enabled and Buffer performs naive malloc() allocation of memory.

When `KAI_TEST_BUFFER_POLICY` is set to one of the above values, the following protections are enabled:

- `PROTECT_UNDERFLOW`: Memory equal to the size of the user buffer rounded to the nearest whole page plus adjacent guard pages is allocated, and the user buffer is aligned to the end of the head guard page thus detecting whenever a buffer underflow occurs.
- `PROTECT_OVERFLOW`: Same as above, but now the edge of the user buffer is aligned to the start of the tail guard page thus detecting whenever a buffer overflow occurs.

Buffer is only intended to opaquely allocate and manage memory. The underlying memory resource can be requested using the familiar `Buffer::data()` method and interacted with using `kai::test::read_array<T>()` and `kai::test::write_array<T>()` utilities.

Signed-off-by: Jakub Sujak <jakub.sujak@arm.com>

Reviewed-by: Viet-Hoa Do <viet-hoa.do@arm.com>
Reviewed-by: Jakub Sujak <jakub.sujak@arm.com>
Reviewed-by: Emil Ohlsson <emil.ohlsson@arm.com>
Approved-by: Felix Johnny Thomasmathibalan <felixjohnny.thomasmathibalan@arm.com>