Enhance certificate decoding in attestation API

The certificate decoding procedure in the attestation API should consider using a more robust implementation, possibly based on CURL lib (see https://curl.se/libcurl/c/curl_unescape.html).

Problem: IAS certificates are downloaded and saved in URL encoded form, then passed the attestation API in C++, where they are decoded. The decoder uses an optimistic procedure, which is not robust. A better approach would require libcurl. However, this is currently not available inside FPC enclaves.