feature: bump cert authorities max size to 20kb

[lrstewart]

Description of changes:

Bumping the cert authorities limit for a customer.

Call-outs:

The extensions are limited to a size of UINT16_MAX, so about 65K. The cert authorities have to share that size with the other extensions, so this max size leaves about 45K for other extensions. We currently only support the cert authorities extension on the certificate request message, which only also supports the signature algorithms and certificate status extensions, but could support more extensions in the future.

Testing:

Existing tests pass, including the test that verifies we can't load the default AL2012 trust store due to its size:

s2n-tls/tests/unit/s2n_cert_authorities_test.c

Lines 99 to 110 in 4a1bfee

	/* Test: too many CAs in trust store */
	if (s2n_cert_authorities_supported_from_trust_store()) {
	DEFER_CLEANUP(struct s2n_config *config = s2n_config_new_minimal(), s2n_config_ptr_free);
	EXPECT_NOT_NULL(config);
	/* This is just a copy of the default trust store from an Amazon Linux instance */
	EXPECT_SUCCESS(s2n_config_set_verification_ca_location(config, S2N_TEST_TRUST_STORE, NULL));
	EXPECT_FAILURE_WITH_ERRNO(
	s2n_config_set_cert_authorities_from_trust_store(config),
	S2N_ERR_TOO_MANY_CAS);
	EXPECT_EQUAL(config->cert_authorities.size, 0);
	};

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