Fixes Issue #1113: TLS: Ignore certs in pem file when loading private key

[jdthomas]

This ignores just certs in the file. Options I considered:

1. Ignore all the non-key items. replacing with _ => {},

2. Ignore explicitly the existing entries in rustls_pemfile::Item::* that are not a key. replacing with rustls_pemfile::Item::X509Certificate(_) | Crl(_) | Csr(_) => {},

3. Ignore just the certs X509Certificate(_) => {}, _ => return Err(TlsConfigError::UnknownPrivateKeyFormat),

Any of those would resolve my issue of certs + keys in same file, happy to adjust diff as needed.

[jdthomas]

Looks like rusttls-pemfile also has a private_key fn similar to the certs fn used at the start of the builder. Perhaps that is what should be used instead of the whole loop?

https://docs.rs/rustls-pemfile/latest/rustls_pemfile/fn.private_key.html

[edit: might look something like this: https://github.com/jdthomas/warp/commit/ff9c9a0892e29259dc7a16cc948a9576129e0039 ]