Public API method for adding resource attributes

[wnstn]

Is your feature request related to a problem? Please describe.
One of the big differentiators for Honeycomb RUM vs other RUM products is our support for wide-data. This will allow users to filter down metrics to the constraints they care about (customers, users, page route owners, etc). Since we will be selling this as a key benefit, we should make it really easy to add attributes that help with that via resource attributes.

Describe the solution you'd like
Ideally a customer can import a method into whatever file has the info they want to attach to all spans and pass in a JS object to that method, and our SDK takes care of attaching all K/Vs in the object as resource attributes.

In an even more ideal world, there's a way to delay span processing until all resource attributes are present, so that devs can attach the attributes they need even if they are not available at the point of SDK instantiation.

Describe alternatives you've considered
N/A

Additional context
I think I've covered everything here, but this request is mostly making sure we think about the DevEx of this common usage pattern.