add log placement support

[josehelps]

The Fastly WAF uses the placement functionality to determine if the logging endpoint is a WAF logging endpoint or not. A WAF logging endpoint is unique in the fact that has unique fields like waf.logdata and waf.rule available to log for one or many WAF event in a single request.

You can find out more information how placement is used for WAF at: https://docs.fastly.com/guides/web-application-firewall/fastly-waf-logging#waf_debug_log

I am not 100% sure if I should update testing for other endpoints, please advice. Also updated the go-fastly library to have the underlying calls for the logging integration also understand placement. Some already supported it but not all, see: fastly/go-fastly#88 for details. Thank you

[lanej]

@d1vious The error should be more obvious now. Placement support in go-fastly needs to be added in other logging providers AND the create/update logging input structs.

[josehelps]

@lanej it does thank you, just created a pull to correct these at fastly/go-fastly#89

[philippschulte]

I'll close this PR in favor of https://github.com/terraform-providers/terraform-provider-fastly/pull/106