Use observables (HTTP requests)

What needs to be changed?

Use native observables with angular's HttpClient.

How is the current state not sufficient?

At the moment we are converting the events got from HttpClient's request method.

• not consistent (it's not how it is done in angular nowadays and modern frontend development)
• easier retry of failed requests (see this)
• use of very useful rxjs operators
• etc. see this

Which changes are necessary?

@openstapps/api should be "aware" of this. it should provide support for native angular's HTTPClient (whose methods use observables)

Do the proposed changes impact current use cases?

Yes. Some refactoring is needed in the data module and other modules using this module.

cc: @abcdev