Add possibility to find certificate chain

[al1img]

For TLS or other certificate verification operations we need not a single certificate but rather the certificate chain. The certificate chain is built by following the issuer->subject path. With the current API the certificate chain can be imported to PKCS11 device but there is no possibility to export it back from PKCS11 device as the current API supports finding only one specific certificate by id, by label, or by serial. It is impossible to find other chained certificates by issuer->subject path.

To solve the issue the find certificate API should be extended to allow finding the certificate by other criteria such as subject and key ID. Or new API that performs the finding certificate chain by issuer->subject path internally should be added to the library.

I've added the proposal PR #83 that implements the second approach (new API to find the certificate chain internally).