[gpg] Use the Bouncy Castle light-weight API directly

No need to go through JCA for just parsing a signature. We can use BC
as a simple library, which makes the code work even if the Bouncy Castle
security provider is not installed.

Change-Id: Ie5609aedfa36aeb3560504cb69a7e89f6583c52d
Signed-off-by: Thomas Wolf <twolf@apache.org>

org.eclipse.egit.core/src/org/eclipse/egit/core/internal/signing/ExternalGpgSigner.java

org.eclipse.egit.core/META-INF/MANIFEST.MF