openssl3 integration: cleanup providers

[toidiu]

Description of changes:

OSSL providers need to be cleaned up after load. This PR iterates over all providers via OSSL_PROVIDER_do_all and calls OSSL_PROVIDER_unload on each. The cleanup function is called before process end in s2n_cleanup_atexit_impl

Testing:

Manual runs of OSSL3 in CI:

a50ee3dbe417b8993e90da662590a891334c6b38

• S2nIntegrationV2SmallBatch
• s2nGeneralBatch

How is this change tested (unit tests, fuzz tests, etc.)? Are there any testing steps to be verified by the reviewer?

Is this a refactor change? If so, how have you proved that the intended behavior hasn't changed?

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[lrstewart]

Nit, kind of silly spacing

Suggested change

	return S2N_RESULT_OK;
	return S2N_RESULT_OK;

[toidiu]

I think its fine. It separates the important code from the return bit. We do this quite often actualy:

s2n-tls/crypto/s2n_libcrypto.c

Lines 71 to 73 in bc51f39

	RESULT_ENSURE(s2n_constant_time_equals((const uint8_t *) expected_version_name, (const uint8_t *) s2n_libcrypto_get_version_name(), (const uint32_t) strlen(expected_version_name)), S2N_ERR_LIBCRYPTO_VERSION_NAME_MISMATCH);
	return S2N_RESULT_OK;

[lrstewart]

We usually don't do it for single line functions :)