Enable providing credentials for HTTP authentication

Enable providing credentials for HTTP authentication

This change is based on http://egit.eclipse.org/r/#change,1652
by David Green. The change adds the concept of a CredentialsProvider
which can be registered for git transports and which is
responsible to return credential-related data like passwords and
usernames. Whenenver the transports detects that an authentication
with certain credentials has to be done it will ask the
CredentialsProvider for this data. Foreseen implementations for
such a Provider may be a EGitCredentialsProvider (caching
credential data entered e.g. in the Clone-Wizzard) or a NetRcProvider
(gathering data out of ~/.netrc file).

Bug: 296201
Change-Id: Ibe13e546b45eed3e193c09ecb414bbec2971d362
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Stefan Lay <stefan.lay@sap.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
CC: David Green <dgreen99@gmail.com>