Add support for callbacks in header values for Simple upload adapter

[Mgsy]

📝 Provide a description of the improvement

Currently, it's possible to define additional headers for the Simple upload adapter with the following config:

simpleUpload: {
	headers: {
		'X-Foo-Bar': 'Value'
	}
}

However, sometimes it might be desired to have the value dynamic (e.g. due to refreshing token), so it would be nice to add support for callbacks. 

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