feat(fetchye): support headers as a function

[code-forger]

Description

Support a way of passing 'dynamic headers'

Motivation and Context

This allows some headers to be created at the time the api call is actually made, including when the run function is called.

This significantly reduces the boiler plate required to otherwise achieve this functionality.

How Has This Been Tested?

Unit tests
No existing tests failing proves this is fully backwards compatible

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (adding or updating documentation)
• Dependency update

Checklist:

• My change requires a change to the documentation and I have updated the documentation accordingly.
• My changes are in sync with the code style of this project.
• There aren't any other open Pull Requests for the same issue/update.
• These changes should be applied to a maintenance branch.
• I have added the Apache 2.0 license header to any new files created.

What is the Impact to Developers Using Fetchye?

New feature

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Latest deployment of this branch, based on commit 520cd97:

Sandbox	Source
quick-install	Configuration
fetchye-provider-install	Configuration
fetchye-redux-provider-install	Configuration
nextjs-fetchye-ssr	Configuration