New interface to retrieve info about running components inside of nodes.

[wildum]

Before ComponentNodes were the only nodes managing a component. The new modules will introduce two new nodes that will have running components inside them:

• the CustomComponentNode will be running a custom component defined by a declare.
• the ImportConfigNode will be running a component to retrieve a module from a given location (the component depends on the source, if the module is stored in a file then the node will run a local_file component).

The component provider API is used to retrieve information about running components. It's implemented by Flow to access the components within the graph. Using the ComponentInfo interface instead of the ComponentNode struct will allow Flow to retrieve information about components running inside any nodes.

Flow services using the API will properly retrieve info about all the components involved in the pipeline. That means, for example in our case, that the UI will be able to show the ImportConfigNode and the CustomComponentNode.

API change:
The Registration field has been replaced by simply the name of the block as a string for the following reasons:

• Registration is used to retrieve information about built-in components. It's not a concept that CustomComponents have.
• Registration provides a "Build" function that is used to build the component. The API is used to retrieve information about running components so this is not relevant information to expose.
• Registration provides Arguments and Exports that are already available in the Info struct. This information is redundant because these two fields are initialized by the Registration when the node is created.
  The only field used (and the only relevant one according to the reasons above) is the name of the block. That's why this PR suggests replacing it with BlockName.

The MarshalJSON has the warning "The format of the representation is not stable and is subject to change." which I believe makes the above change ok.

[rfratto]

LGTM after this final set of comments :)

[rfratto]

Let's go!