Create XxxFields struct for grouped IEs with many children

[wmnsk]

Currently we need to iterate over children to find specific IEs and to get its values, which is a bit painful considering the number of children. Adding a struct for them and set values directly to its fields would make it easier for users.

cf. README

type CreatePDRFields struct {
	PDRID uint16
	FARID uint32
	...
}

Here is the list of IEs defined in Table 7.x instead of Clause 8.x in TS 29.244

• 1 Create PDR
• 2 PDI
• 3 Create FAR
• 4 Forwarding Parameters
• 5 Duplicating Parameters
• 6 Create URR
• 7 Create QER
• 8 Created PDR
• 9 Update PDR
• 10 Update FAR
• 11 Update Forwarding Parameters
• 12 Update BAR (PFCP Session Report Response)
• 13 Update URR
• 14 Update QER
• 15 Remove PDR
• 16 Remove FAR
• 17 Remove URR
• 18 Remove QER
• 51 Load Control Information
• 54 Overload Control Information
• 58 Application ID's PFDs
• 59 PFD context
• 68 Application Detection Information
• 77 Query URR
• 78 Usage Report (Session Modification Response)
• 79 Usage Report (Session Deletion Response)
• 80 Usage Report (Session Report Request)
• 83 Downlink Data Report
• 85 Create BAR
• 86 Update BAR (Session Modification Request)
• 87 Remove BAR
• 99 Error Indication Report
• 102 User Plane Path Failure Report
• 105 Update Duplicating Parameters
• 118 Aggregated URRs
• 127 Create Traffic Endpoint
• 128 Created Traffic Endpoint
• 129 Update Traffic Endpoint
• 130 Remove Traffic Endpoint
• 132 Ethernet Packet Filter
• 143 Ethernet Traffic Information
• 147 Additional Monitoring Time
• 165 Create MAR
• 166 3GPP Access Forwarding Action Information
• 167 Non-3GPP Access Forwarding Action Information
• 168 Remove MAR
• 169 Update MAR
• 175 Update 3GPP Access Forwarding Action Information
• 176 Update Non 3GPP Access Forwarding Action Information
• 183 PFCP Session Retention Information (within PFCP Association Setup Request)
• 187 User Plane Path Recovery Report
• 189 Join IP Multicast Information IE within Usage Report
• 190 Leave IP Multicast Information IE within Usage Report
• 195 Created Bridge Info for TSC
• 199 TSC Management Information IE within PFCP Session Modification Request
• 200 TSC Management Information IE within PFCP Session Modification Response
• 201 TSC Management Information IE within PFCP Session Report Request
• 203 Clock Drift Control Information
• 205 Clock Drift Report
• 211 Remove SR
• 212 Create SR
• 213 Update SR
• 214 Session Report
• 216 Access Availability Control Information
• 218 Access Availability Report
• 220 Provide ATSSS Control Information
• 221 ATSSS Control Parameters
• 225 MPTCP Parameters
• 226 ATSSS-LL Parameters
• 227 PMF Parameters
• 233 UE IP address Pool Information
• 238 GTP-U Path QoS Control Information
• 239 GTP-U Path QoS Report (PFCP Node Report Request)
• 240 QoS Information in GTP-U Path QoS Report
• 242 QoS Monitoring per QoS flow Control Information
• 247 QoS Monitoring Report
• 252 Packet Rate Status Report
• 254 Ethernet Context Information
• 255 Redundant Transmission Parameters
• 256 Updated PDR
• 261 Provide RDS configuration information
• 263 Query Packet Rate Status IE within PFCP Session Modification Request
• 264 Packet Rate Status Report IE within PFCP Session Modification Response
• 267 UE IP Address Usage Information
• 270 Redundant Transmission Forwarding Parameters
• 271 Transport Delay Reporting
• 272 Partial Failure Information
• 275 RAT Type
• 276 L2TP Tunnel Information
• 277 L2TP Session Information
• 279 Created L2TP Session
• 290 PFCP Session Change Info
• 295 Direct Reporting Information
• 300 MBS Session N4mb Control Information
• 301 MBS Multicast Parameters
• 302 Add MBS Unicast Parameters
• 303 MBS Session N4mb Information
• 304 Remove MBS Unicast Parameters
• 310 MBS Session N4 Control Information
• 311 MBS Session N4 Information
• 316 DSCP to PPI Control Information