Support openstacksdk-style service default microversions from clouds.yaml

[Sharpz7]

Context

Consumers that use clouds.yaml may want per-cloud service microversion defaults using the openstacksdk convention:

clouds:
  cloud_a:
    auth: ...
    compute_default_microversion: "2.87"
  cloud_b:
    auth: ...
    compute_default_microversion: "2.79"

This is useful for long-running multi-cloud processes where environment variables such as OS_COMPUTE_API_VERSION are too coarse because they apply process-wide rather than per selected cloud.

One downstream example is openstack-exporter, which can scrape multiple clouds from one clouds.yaml and currently has to manage Nova microversions outside of clouds.yaml: openstack-exporter/openstack-exporter#500

Current behavior

clientconfig.Cloud exposes typed API-version override fields such as identity_api_version and volume_api_version, but does not expose compute_default_microversion or a generic {service_type}_default_microversion mechanism. LoadCloudsYAML() unmarshals into map[string]Cloud, so callers cannot recover these keys through the public Cloud type after parsing.

Proposal

Add support for openstacksdk-style service default microversions in clientconfig, preferably in a way that can support more than Nova over time.

Possible API shapes:

• a generic map on Cloud, e.g. DefaultMicroversions map[string]string, populated from keys matching {service_type}_default_microversion;
• typed fields for known OpenStack services, starting with ComputeDefaultMicroversion;
• helper methods such as cloud.DefaultMicroversion("compute") so downstream consumers do not need to duplicate key normalization rules.

Open questions:

• Should this live only in github.com/gophercloud/utils/v2/openstack/clientconfig, or should github.com/gophercloud/gophercloud/v2/openstack/config/clouds grow equivalent support too?
• Should service names follow catalog service types exactly, such as compute, block-storage, and shared-file-system, or openstacksdk config aliases?
• Should env var precedence remain the caller's responsibility, or should clientconfig.NewServiceClient apply a parsed default microversion automatically where possible?

[Sharpz7]

Draft implementation opened: #288

Companion parser change in gophercloud/gophercloud: gophercloud/gophercloud#3881