During internal trialing of context provider plugins, we discovered that
the JSON schema validation of the Cloud Assembly is throwing soot into
our food: any properties we add aren't exactly those of another context
provider, and therefore not allowed.

Add a new branch of the context provider schema, a `PluginContextQuery`,
which allows any set of properties. The actual plugin name is now merely
a property passed to the "plugin" context provider.

When the handler is registered or the query is handled, we combine the
key in the lookup table with the actual provider name as such:
`plugin:${pluginName}`. This is an implementation detail of the CLI, and
not part of the contract (can be changed easily whenver necessary).