Add configurable side channels mitigations; enable them on soft AES (#13739)

* Add configurable side channels mitigations; enable them on soft AES

Our software AES implementation doesn't have any mitigations against
side channels.

Go's generic implementation is not protected at all either, and even
OpenSSL only has minimal mitigations.

Full mitigations against cache-based attacks (bitslicing, fixslicing)
come at a huge performance cost, making AES-based primitives pretty
much useless for many applications. They also don't offer any
protection against other classes of side channel attacks.

In practice, partially protected, or even unprotected implementations
are not as bad as it sounds. Exploiting these side channels requires
an attacker that is able to submit many plaintexts/ciphertexts and
perform accurate measurements. Noisy measurements can still be
exploited, but require a significant amount of attempts. Wether this
is exploitable or not depends on the platform, application and the
attacker's proximity.

So, some libraries made the choice of minimal mitigations and some
use better mitigations in spite of the performance hit. It's a
tradeoff (security vs performance), and there's no one-size-fits all
implementation.

What applies to AES applies to other cryptographic primitives.

For example, RSA signatures are very sensible to fault attacks,
regardless of them using the CRT or not. A mitigation is to verify
every produced signature. That also comes with a performance cost.
Wether to do it or not depends on wether fault attacks are part of
the threat model or not.

Thanks to Zig's comptime, we can try to address these different
requirements.

This PR adds a `side_channels_protection` global, that can later
be complemented with `fault_attacks_protection` and possibly other
knobs.

It can have 4 different values:

- `none`: which doesn't enable additional mitigations.
"Additional", because it only disables mitigations that don't have
a big performance cost. For example, checking authentication tags
will still be done in constant time.

- `basic`: which enables mitigations protecting against attacks in
a common scenario, where an attacker doesn't have physical access to
the device, cannot run arbitrary code on the same thread, and cannot
conduct brute-force attacks without being throttled.

- `medium`: which enables additional mitigations, offering practical
protection in a shared environement.

- `full`: which enables all the mitigations we have.

The tradeoff is that the more mitigations we enable, the bigger the
performance hit will be. But this let applications choose what's
best for their use case.

`medium` is the default.

Currently, this only affects software AES, but that setting can
later be used by other primitives.

For AES, our implementation is a traditional table-based, with 4
32-bit tables and a sbox.

Lookups in that table have been replaced by function calls. These
functions can add a configurable noise level, making cache-based
attacks more difficult to conduct.

In the `none` mitigation level, the behavior is exactly the same
as before. Performance also remains the same.

In other levels, we compress the T tables into a single one, and
read data from multiple cache lines (all of them in `full` mode),
for all bytes in parallel. More precise measurements and way more
attempts become necessary in order to find correlations.

In addition, we use distinct copies of the sbox for key expansion
and encryption, so that they don't share the same L1 cache entries.

The best known attacks target the first two AES round, or the last
one.

While future attacks may improve on this, AES achieves full
diffusion after 4 rounds. So, we can relax the mitigations after
that. This is what this implementation does, enabling mitigations
again for the last two rounds.

In `full` mode, all the rounds are protected.

The protection assumes that lookups within a cache line are secret.
The cachebleed attack showed that it can be circumvented, but
that requires an attacker to be able to abuse hyperthreading and
run code on the same core as the encryption, which is rarely a
practical scenario.

Still, the current AES API allows us to transparently switch to
using fixslicing/bitslicing later when the `full` mitigation level
is enabled.

* Software AES: use little-endian representation.

Virtually all platforms are little-endian these days, so optimizing
for big-endian CPUs doesn't make sense any more.

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