WASM: Add [store_exists] host functions

Context

This is a more efficient alternative to store_has, which has shown very bad performances impact on realistic kernels. Though a patch is being made by Tarides, having this more straightforward way to know the existence of keys remains a good idea for the short term, and it is more than probably that its performances will be better than store_has even after the fix.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR