Add `Organization` as a first-class field to `WorkspaceFilter` so Go SDK
callers can filter workspaces by organization name or UUID without
constructing a raw `FilterQuery` string.
The backend already supports `organization:` as a search parameter via
`searchquery.Workspaces()`. This change surfaces it consistently
alongside the existing `Owner`, `Template`, and `Status` fields.
Closes https://github.com/coder/coder/issues/21545
