Alternative Endpoint for x509.ParsePKIXPublicKey that allows non-fatal errors

I have to parse a P-192 public key and although it is [not implemented](https://github.com/golang/go/issues/41035) in the upstream `x509` package, this repo is able to parse it properly and flags the fact that it is an insecure algorithm as a non-fatal error; however, the `ParsePKIXPublicKey` function [makes these non-fatal errors fatal errors](https://github.com/google/certificate-transparency-go/blob/7710282e49162cbd95c500777522f436fd5fc279/x509/x509.go#L110-L113), preventing the parsing of a P-192 key. The comment mentions that this is "for this entrypoint"; however, I don't see another entry point that would allow for more lax parsing of these public keys.

Any ideas on an alternative entrypoint for parsing these keys?

Start by reading x509/x509.go around ParsePKIXPublicKey and its handling of non-fatal errors at lines 110-113, then review upstream issue 41035 for the P-192 limitation. Define an alternative parsing entry point that permits the repository's non-fatal algorithm errors without changing the existing entry point's behavior.

Written by the indexing model from the issue text.