RESTClient: update the template for `dlt init` to use RESTClient

[burnash]

Background

dlt init <source> <destination> initializes a new <source>. When <source> is not found in the verified-sources repo it uses one of the two templates:

• https://github.com/dlt-hub/verified-sources/blob/master/init/pipeline.py (default)
• https://github.com/dlt-hub/verified-sources/blob/master/init/pipeline_generic.py (for the case of dlt init --generic)

Both of the templates above use dlt.sources.helpers.requests to make HTTP calls. The task is to convert both templates to use RESTClient instead of dlt.sources.helpers.requests.

[rudolfix]

@burnash this should be moved to verified sources repo once assigned, right?

[burnash]

TODO: Update the pipeline tutorial

[sultaniman]

@burnash I will use pokemon api to showcase pagination for --generic template