This commit adds `TranslationSources` to the cluster
`ResizeInstruction`. These are the sources of translation
partitions which the receiving node needs in order to support
partition distribution in the new, resized cluster.
This also fixes a bug where index options were not being
encode in the proto Index object. That meant that the schema
transferred via protobuf was not correct. The reason why
things normally worked is because index creation typically
happens on the CreateIndex message, which does include the
options.
TODO:
- [ ] implement the TranslateStore interface for `InMemTranslateStore`
and `mock.TranslateStore`
- [ ] surely need some more tests around the `ReadFrom` and `WriteTo`
