loadbalancer: add support for provider flavor and availability zone capabilities #3931

What is missing?

Hi maintainers,

The openstack/loadbalancer/v2/providers package is currently supporting listing enabled provider drivers below,

GET /v2/lbaas/providers

and current Octavia exposes two additional read only provider APIs that are not currently available in Gophercloud yet

GET /v2/lbaas/providers/{provider}/flavor_capabilities
GET /v2/lbaas/providers/{provider}/availability_zone_capabilities

The flavor capabilities API was introduced in octavia API version 2.6: Octavia API reference

and the availability zone capabilities API was also introduced in version 2.14: Octavia API reference

I encountered this while adding Octavia provider validation(using amphora provider) and diagnostics to an openstack based kubernetes Gateway API controller. so without SDK support, callers currently need to send HTTP requests on these URLs manually.

If this feature proposal approach looks reasonable, i would be happy to work on this implementation also with acceptance tests.