Allow MetaData to store dictionaries and arrays

#### Feature Description
Is there a reason the metadata KV store is limited to strings?

I'd really like to be able to also store arrays or dictionaries containing more metadata about a given service.

#### Use Case(s)

- Storing a list of authors for a component
- Storing dependents and their versions

We use the register meta section to store descriptions for a connected service, but have to store other info in a database instead.

Start by locating Consul's metadata KV store and register meta entry points; no files or tests are identified in the issue. Review how string values are represented and validated, then define compatible array and dictionary handling for the listed author and dependent-version use cases, with tests covering the new metadata shapes.

Written by the indexing model from the issue text.