Add update organization member request api call

[albert-kerr]

Based off this documentation https://dev.splunk.com/observability/reference/api/organizations/latest, I implemented the api call

[leandropadua]

@atoulme could you please have a look at this PR? It's a simple change. In Atlassian we use this API to have a workaround to map Active Directory groups to signalfx roles. It would be nice if we could use the native Signalfx go library instead of maintaining our own fork. Thanks