Handle extra headers in PEM encoded private key files

[jkakavas]

Openssl adds some extra headers ( Bag Attributes and Key Attributes ) to keys that it exports from PKCS#12 containers, i.e

Bag Attributes
    friendlyName: mykey
    localKeyID: 54 69 6D 65 20 31 35 33 30 30 32 31 38 30 36 37 31 30 
Key Attributes: <No Attributes>

This information is redundant as the notion of Bags is relevant only for PKCS#12 and can be disregarded, so we should ensure that PemUtils can read private key files even when this is present in a PEM key file.

[elasticmachine]

Pinging @elastic/es-security

[jkakavas]

Bags and attributes are discussed in https://tools.ietf.org/html/rfc7292 . Openssl seems to support adding the Bag Attributes and Key Attributes from the safe bug and the cert bag:

https://github.com/openssl/openssl/blob/7d38ca3f8bca58bf7b69e78c1f1ab69e5f429dff/apps/pkcs12.c#L670

but there seems to be no specification that explicitly defines how these attributes are to be presented in a key/cert file.
RFC7468 allows only whitespace before the header (and that only in Lax mode ), but both Openssl and BouncyCastle allow arbitrary content before the header of PEM files.

I suggest we, too, err a little on the side of interoperability and backwards compatibility with pre-6.4 configurations in the expense of strict conformance to the RFC by allowing and disregarding extra content before the header