Also adds new types to weldr/json.go to support them.
ComposeEntry had to be duplicated instead of used as-is because it
enforces image type strings that do not match what the API uses (the API
types are all lower case, the internal names are capitalized).
TOML output of the source wasn't using the correct field names or case.
This adds toml tags to the weldr.SourceConfigV0, and store.SourceConfig
structs.
