Provide default headers including user-agent for all provider DAGs

[stacimc]

Problem

Some provider APIs require a User-Agent header, like rawpixel (#3679). In rawpixel's case, we only discovered this when the DAG stopped working.

Description

We could set the User-Agent as a default header on all of our provider APIs by setting it in the ProviderDataIngester. Note that currently, headers is empty on the base class and subclasses totally overwrite it when they want to add headers. We would need to update the base class to provide default headers but still allow subclasses to extend them.