Expose the api to add/remove metadata and artifacts to trrack nodes

[kirangadhave]

We have the types and the metadata attributes on the nodes. We don't expose a function to add/remove/set these.

Metadata is the ONLY change allowed in an already created node. To keep the provenance of metadata changes, the metadata attribute should have the following data structure:

type MetadataObject<T> = {
    timestamp: string;
    value: T
}

type Metadata<T> = Array<MetadataObject<T>>

The API should provide the following:

• [x] Add new metadata to the node. Only take the metadata as input, and generate timestamps internally.
• [x] Get available metadata types
• [x] Get the latest metadata of a type
• [x] Get all metadata of a type
• [x] Get all metadata

Implement a similar API for artifacts. Artifacts can be used to store arbitrary objects/object URLs like screenshots, etc.

[kirangadhave]

If adding proper type safety using generics, breaks the current trrack version, I will use any for handling metadata types. Next major release will include the actual typings.