feat: Add PrimaryKey implementation example for custom types

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Why?

Sometimes, it's useful to store a custom type as a key in a Map storage structure. To do this, users need to implement proper serialization for such types.

So I found it useful to add such an example with a reference implementation for a widely used Denom enum that represents either a native token or a cw20 token.

Scope:

• Made some util functions as the public that can be helpful in custom types implementations
• Added example test with primary key implementation for `Denom
• Added a readme reference for that example

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Question for maintainers: From the selectivity perspective, mb, is it better to write the value first as a prefix and after its type (which is either 1 or 2 in our case)?