feat: Add support for pkcs#8 private keys

[Andrew-Lees11]

This pull request adds support for PKCS#8 formatted private keys on macOS.
These are keys that begin with:

-----BEGIN PRIVATE KEY-----

This has been achieved by stripping the header byes for private keys inside stripX509CertificateHeader.

A test for this format has been added which would fail before but will now succeed.

This is required because on Swift-JWT issue 51 somebody was providing a pkcs8 private key which was working on linux but not on macOS.

[billabt]

CI is failing... Don't want to merge unless we can get a clean CI build. I restarted it but it's still failing on the Swift dev snapshot. Here's that error being reported...

>> Building Swift package...
/Users/travis/build/IBM-Swift/BlueRSA: error: manifest parse error(s):
<unknown>:0: error: Swift does not support the SDK 'MacOSX10.14.sdk'
The command "./Package-Builder/build-package.sh -projectDir $TRAVIS_BUILD_DIR" exited with 1.

It looks to me like it could be a travis configuration issue...

[ianpartridge]

Fixed the CI problem by rebasing the branch onto master so it included 9ead942 (the problem is that the latest Swift snapshots will only run on Xcode 11, which isn't available in Travis yet 😢)

@billabt can you approve and merge this please?

[billabt]

👍