[Bug] - can't create data streams in serverless projects

[Kushmaro]

** background **
In serverless projects ILM policies are removed in favor of data streams.
In our terraform examples, to create a data stream, we require you to create an ILM policy (and that's our docs example)

** problem **
Since our data stream resource implements most properties as "computed' i.e read only - one can not create a data stream currently on a serverless project, using the elasticstack provider.

** likely the solution **
enable the properties required to create a data stream via the API as not read-only and allow user input to the data stream resource