[WIP] Add support for unified hosts

[jh-db]

What changes are proposed in this pull request?

This PR adds support for unified hosts, i.e. hosts that support both workspace and account methods. To do this, it:

• separates client type from host type determination.
• adds an isUnifiedHost attribute to config to indicate whether a host is unified
• adds a WorkspaceId attribute to config, which is necessary for workspace clients that talk to unified hosts
• adds UnifiedOAuthArgument, which is used in endpoint_supplier.go to discover OAuth endpoints on unified hosts and cache tokens for those hosts.
• Adds a request visitor which adds an X-Databricks-Org-Id header to requests made by workspace clients

How is this tested?

Tests to be added after discussion.

[github-actions]

Please ensure that the NEXT_CHANGELOG.md file is updated with any relevant changes.
If this is not necessary for your PR, please include the following in your PR description:
NO_CHANGELOG=true
and rerun the job.

[github-actions]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger:
go/deco-tests-run/sdk-go

Inputs:

• PR number: 1307
• Commit SHA: cf529302f176c6ba669df5b81d0c1097be6fb543

Checks will be approved automatically on success.