High-Level OpenPGP API

[vanitasvitae]

This PR contains a high-level API for OpenPGP, which simplifies the following tasks:

• Message Creation
  • OpenPGPMessageGenerator sets up an OpenPGPMessageOutputStream based on user configuration
• Message Consumption
  • OpenPGPMessageProcessor processes encrypted / signed / compressed OpenPGP messages and emits the plaintext along with metadata
• Certificate / Key Evaluation
  • OpenPGPCertificate acts as high-level API pendant to PGPPublicKeyRing, evaluating expiration / revocation / key signatures and allows the user to get verified information about the cert
  • OpenPGPKey provides similar high-level API pendant to PGPSecretKeyRing

I decided to use a common OpenPGP* naming scheme for the new HighLevel API classes and also decided to make use of general terminology from the book "OpenPGP for application developers" (https://openpgp.dev).

This PR is still pretty much very early work in progress (as is #1857, which will eventually integrate with this API), but I want you to be able to comment on it during the development phase already :)

[vanitasvitae]

This PR is now based on #1926

[vanitasvitae]

This PR contains #1931 and may need to be rebased if the other patch is merged.

[vanitasvitae]

@dghgit I noticed, that in many places you replaced

for (item : collection)
{

with

for (Iterator it = collection.iterator(); it.hasNext();)
{
    Item item = iterator.next();

I was under the impression that BC was now targeting Java 1.8 primarily, which already supports the former method of collection iteration.
Am I missing something? Why are the Iterators not using generics?

Edit: I see you recently made some changes for Java 4 and 5 compatibility.

[vanitasvitae]

The PR is ready for review. Let me know if you like me to squash it for you :)

[vanitasvitae]

As another cherry on top, I present to you bc-sop, which implements the stateless OpenPGP protocol as a native-image using Bouncy Castle :)