Change the JSON generation method

[notJoon]

Description

Functions called via API internally use the gno.land/p/demo/json package to generate JSON. This package operates by directly adding required fields to json.Node objects when creating JSON and then encoding them.

Looking at the current JSON generation approach in our codebase, since required fields are called and generagted within each function as needed, there is a lot of redundancy and many sections with poor readability and maintainability. In my opinition, it would better align with this package's intended usage to create functions that generate json.Node for each type and then assemble them using the builder pattern (supported by the package as json.Builder).

Additionally, there are cases where numeric and boolean values are converted to strings, before output during JSON generation. We could reduce unnecessary operations by outputting these values in their original types without conversion.

Furthermore, when comparing JSON output values in tests, we're currently using direct string comparison, which significantly reduces test maintainability. This issue can be resolved by using the methods provides by JSON package (refer to the `json/nodes.gno file).

Prior Arts

1. feat(examples): forms #2 gnolang/gno#3524 (comment)
2. https://github.com/gnoswap-labs/gnoswap/blob/main/contract/r/gnoswap/launchpad/json_builder.gno

[dongwon8247]

Note that we might need to change the current usage of the JSON package to Gno's native JSON encoder when it's introduced before mainnet.