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