Add getters for private default Headers and authentications

Created by: NANASHI0X74

Rationale for this is: At the moment I'm in a situation where code generation (or rather the scheme generation for our backend code) doesn't generate code that works with our backend, so I need to do one specific request by hand. I'd like to still use the same default headers and authentications that the Apiclient uses though.

This is a really minor change, so I thought I'd show you the code before talking. Submitting this as draft so I can do the build and sample generation later

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  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

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mentioned in issue #8510 (closed)

changed title from add getters for private default Headers and authentications to Add getters for private default Headers and authentications

added Client: Dart Enhancement: General labels

changed milestone to %5.0.1

closed