fix: mutable self required for dispatchable ink! methods

[chungquantin]

In our current pop-api contracts, we do not mark self as mutable because the contract storage is not being modified directly; instead, the runtime storage is mutated by the pop-api. However, on the client side, when using libraries that support ink! (such as PJS or Dedot), these methods are perceived as query rather than tx, as the contract itself does not undergo changes.

While it's unclear whether pop-api will be utilized in immutable methods in production, it is crucial to clearly state this requirement in the documentation. This will help ensure that developers understand how pop-api behaves and how it might impact the perception and execution of methods in client-side libraries.

cc: @Daanvdplas @peterwht

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 34.08%. Comparing base (6482a43) to head (c10f15c).
Report is 1 commits behind head on daan/api.

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  Files            34       34           
  Lines          3013     3013           
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  Hits           1027     1027           
  Misses         1954     1954           
  Partials         32       32           

[Daanvdplas]

Nice find!