Add a GpgSignatureVerifier interface, plus a factory to create
instances thereof that is provided via the ServiceLoader mechanism.

Implement the new interface for BouncyCastle. A verifier maintains
an internal LRU cache of previously found public keys to speed up
verifying multiple objects (tag or commits). Mergetags are not handled.

Provide a new VerifySignatureCommand in org.eclipse.jgit.api together
with a factory method Git.verifySignature(). The command can verify
signatures on tags or commits, and can be limited to accept only tags
or commits. Provide a new public WrongObjectTypeException thrown when
the command is limited to either tags or commits and a name resolves
to some other object kind.

In jgit.pgm, implement "git tag -v", "git log --show-signature", and
"git show --show-signature". The output is similar to command-line
gpg invoked via git, but not identical. In particular, lines are not
prefixed by "gpg:" but by "bc:".

Trust levels for public keys are read from the keys' trust packets,
not from GPG's internal trust database. A trust packet may or may
not be set. Command-line GPG produces more warning lines depending
on the trust level, warning about keys with a trust level below
"full".

There are no unit tests because JGit still doesn't have any setup to
do signing unit tests; this would require at least a faked .gpg
directory with pre-created key rings and keys, and a way to make the
BouncyCastle classes use that directory instead of the default. See
bug 547538 and also bug 544847.

Tested manually with a small test repository containing signed and
unsigned commits and tags, with signatures made with different keys
and made by command-line git using GPG 2.2.25 and by JGit using
BouncyCastle 1.65.

Bug: 547751
Change-Id: If7e34aeed6ca6636a92bf774d893d98f6d459181
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

Motivation: BouncyCastle serves as 'default' implementation of
the GPG Signer. If a client application does not use it there is no need
to pull in this dependency, especially since BouncyCastle is a large
library.

Move the classes depending on BouncyCastle to an OSGi fragment extending
the org.eclipse.jgit bundle. They are moved to a distinct internal
package in order to avoid split packages. This doesn't break public API
since these classes were already in an internal package before this
change.

Add a new feature org.eclipse.jgit.gpg.bc to enable installation. With
that users can now decide if they want to install it.

Attempts to sign a commit if org.eclipse.jgit.gpg.bc isn't available
will result in ServiceUnavailableException being thrown.

Bug: 559106
Change-Id: I42fd6c00002e17aa9a7be96ae434b538ea86ccf8
Also-by: Michael Dardis <git@md-5.net>
Signed-off-by: Michael Dardis <git@md-5.net>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>
Signed-off-by: David Ostrovsky <david@ostrovsky.org>