Add support of Flavors and FlavorProfiles for Octavia

[Pyjou]

I propose to add support of the Flavors and FlavorProfiles endpoints for Octavia in Gophercloud.

https://docs.openstack.org/api-ref/load-balancer/v2/index.html#flavors
https://docs.openstack.org/api-ref/load-balancer/v2/index.html#flavor-profiles

[github-actions]

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