Add support for verifying PKCS7 signed attributes

[samuel40791765]

Issues:

Resolves CryptoAlg-2946

Description of changes:

I discovered this while trying to fix our PKCS7 implementation to use indefinite encoding. The PKCS7 file that Ruby's PKCS7 tests uses signed attributes, but PKCS7_verify bails out whenever it encounters files that have signed attributes. There are still other issues with the Ruby PKCS7 test that we'll have to fix (indefinite length ASN1), but I believe we should fix the missing support for verifying signed attributes first.
AWS-LC turns on PKCS7_NOATTR by default in PKCS7_sign, so our existing PKCS7_verify implementation can do a successful sign/verify round trip against itself. However, OpenSSL does not turn on PKCS7_NOATTR by default and signed attributes are added automatically to the PKCS7 file if no flags are set. This means that the current state of AWS-LC's PKCS7_verify would fail against files generated by the default of OpenSSL's PKCS7_sign. This PR adds support for verifying PKCS7 signed attributes to fix the misalignment.

Call-outs:

N/A

Testing:

1. Test file from Ruby. The intention is to build upon the test to fix the ASN1 indefinite length issues later on.
2. Test file generated by OpenSSL. Our PKCS7_sign does not support signed attributes, so we can only test against generated files by OpenSSL.

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