spec,libcni: add metadata field to Interface struct for CNI Result

[MikeZappa87]

This PR resolves: #1050

This PR adds an optional 'metadata' field (map[string]string) to the Interfaces struct.

type Interface struct { 
    Name         string `json:"name"` 
    Mac          string `json:"mac,omitempty"` 
    Sandbox      string `json:"sandbox,omitempty"` 
    Metadata     map[string]string `json:"metadata,omitempty"` 
} 

[coveralls]

coverage: 70.897%. remained the same
when pulling 6b248ea on MikeZappa87:improvement/metadataoninterface
into 2317778 on containernetworking:main.

[tamilmani1989]

@MikeZappa87 shall we mark this PR as draft until we close on #1050