Fix PQC and hybrid certificate regressions

[Frauschi]

Hi all,

Due to recent changes in the logic to decode private keys and to parse the TLS1.3 CertificateVerify message, some regressions regarding PQC private keys and hybrid certificates have been introduced:

• Decoding PQC private keys failed, as the PKCS8 header of a decoded DER file is now already removed before parsing the key.
• The key size wasn't properly stored in the context for PQC keys after decoding a certificate (always the maximum size)
• The two 16-bit size values in case of a hybrid signature in the CertificateVerify message have been incorrectly decoded as 32-bit values instead of 16-bit values. This resulted in wrong values, leading to segmentation faults.

All three regressions are fixed with the changes in this commit.

[wolfSSL-Bot]

Can one of the admins verify this patch?

[dgarske]

Thanks @Frauschi. Okay to test. Contributor agreement on file.

[dgarske]

@anhu please regression test this PR #7576 and #7577 ?

[anhu]

Tested. Works as expected.