[NEXUS-5741] Extract non maven specific file content validation to super class to be able to reuse it

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5741] Implement a p2 specific file content FileTypeValidator (including Pack2000 validator)

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5741] Configure P2 repositories to use FileTypeItemContentValidator

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Replace plexus with jsr330 and use static IDs for validator hints

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>