Bug 6337 - Handle string route-key from App Routes when Binary route-key is expected

Bug 6337 - Handle string route-key from App Routes when Binary route-key is expected

Change binary typed route keys to string. Modify RIB support extensions to
encode key with Base64.

The binary type is decoded with Base64, where length is restricted.
The BGP application RIB allows user to originate a route
and binary typed route key is limiting user defined route keys to a
specific length (cut-off when does not fit).
Also using plain text for binary typed route keys is not correct.

The route-key type change is invisible in restconf output.

Change-Id: Ieab24705ceebe0b527b4ddae81a4109c9c250de0
Signed-off-by: Milos Fabian <milfabia@cisco.com>
(cherry picked from commit eca666c067d809426fcb5936d559b3e08271f48f)