Suggestion: Surface storage costs i a distinct function

Hey!
Since I could not find the current storage costs for mutable and immutable Storage in the docs I was looking at the SDK expecting to find a `getUploadPriceFromBytes ` function or similar like I am used to using the Irys / Bundlr Umi plugin.

I would suggest to add a function like this to allow your users to calculate the costs up front.

Thank you!