Validate chosen PEM or JKS file is actually a PEM or JKS file

[jlrobins]

We could do a little file validation here based on the file type chosen and sniffing of contents through basic heuristics to help ensure the user has chosen not obviously definitely wrong files based on either JKS or PEM formats.

@noeldevelops could do the form -> extension host messaging easy enough, and a @jlrobins or @shouples could provide the implementation functions for extension-side validateSSLFileFormat(type: PEMorJKSEnum, path: Path): void or Error-with-message-to-show-to-user-raised function.

[rohitsanj]

We also have PKCS12 in case we wanted to validate that too.