iRule lookup IP address from remote json endpoint and whitelist source traffic?

you can use "send" function in irules.
you can use it to send http request, but not https, interrogation to other server.

https://clouddocs.f5.com/api/irules/send.html

if the ip list changes is not frequent,
you can sftp the list file to f5
and schedule (using crontab) to run tmsh command to update datagroup
then use the datagroup in local traffic policy or irules to do ip filtering.

https://clouddocs.f5.com/cli/tmsh-reference/v16/modules/ltm/ltm_data-group_external.html