Change parameter string types to bytes type

There has been identified a problem when Protobuf string type is used: this
type does not support non-UTF8 characters and this limitation can cause
Protobuf encoding/decoding failure of an X2 parameter with arbitrary binary
value. To fix the issue replace string types of all X2 parameters in Protobuf
files with bytes type to support wider value range. Parameters
'protobuf_revision' and 'gNbID' types in x2ap_streaming.proto file were still
left a string type, because undoubtedly these parameter values are always
strings.

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: I45f1631e3e3e96c408c7d2c015221ef8e2857608