net: lwm2m: SenML-CBOR Encoder and CDDL update

Added support for historical data encode by adding base time (bt)
and time (t) label. New labels are needed for Encoder so Decoder
is not regenerated.

Added support for SenML-CBOR to write time series data.
Use "bt" base time and "t" timestamp labels for data cache.

Signed-off-by: Juha Heiskanen <juha.heiskanen@nordicsemi.no>

subsys/net/lib/lwm2m/lwm2m_senml_cbor.cddl

@@ -2,13 +2,15 @@ lwm2m_senml = [1* record]
 
 record = {
 	? bn => tstr,            ; Base Name
+	? bt => int .size 8,     ; Base Time
 	? n => tstr,             ; Name
+	? t => int .size 8,      ; Time
 	? ( vi => int .size 8 // ; Integer Value
 	    vf => float       // ; Float Value
 	    vs => tstr        // ; String Value
 	    vb => bool        // ; Boolean Value
 	    vd => bstr ),        ; Data Value
-	0*3 key-value-pair       ; To handle unordered maps; length-first ordered map keys
+	0*5 key-value-pair       ; To handle unordered maps; length-first ordered map keys
 }
 
 ; now define the generic versions
@@ -17,7 +19,9 @@ key-value-pair = ( int => value )
 value = tstr / bstr / int .size 8 / float / bool
 
 n  = 0
+t = 6
 bn = -2
+bt = -3
 vi = 2
 vf = 2
 vs = 3