refactor: use pointer receivers consistently for 08-wasm keeper methods

[VolodymyrBg]

Description

This PR makes the receiver types consistent across keeper method definitions in the light-clients/08-wasm module. All methods now use pointer receivers instead of a mix of pointer and value receivers.

closes: #6165

Changes

• Updated all Keeper methods in keeper.go to use pointer receivers
• Updated QueryPlugins methods in querier.go to use pointer receivers
• Updated test wrapper methods in export_test.go to use pointer receivers

---

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[faddat]

thank you for improving consistency!

It's super helpful!