docs(adr): ADR-21: Subspace specific custom fee tokens

[dadamu]

Description

Closes: #XXXX
This PR introduces the subspace specific custom fee tokens by the ADR.

---

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[RiccardoM]

I have some doubts on the proposed implementation. I think there's no need for multiple messages and steps. Everything could be simplified to use on-chain proposals instead.

[RiccardoM]

Why is this needed? Each subspace will have its own allowed_fee_tokens list anyway. There's no need to have a module-wide list

[dadamu]

Since x/fees may conflict with subspace fee token checks, I think we can update the logic of CheckFees only check dsm. What do you think? @RiccardoM @manu0466

[RiccardoM]

I think we should completely drop the x/fees module instead. It's never been used anyway

[dadamu]

@RiccardoM Updated docs.

[RiccardoM]

Suggested change

	The solution outlined above is **not** backwards compatible and will require a migration script to update all existing subspaces to the new version. This script will handle the following tasks:
	- migrate all subspaces to have a default allowed fee tokens list.
	The solution outlined above is **not** backward compatible, and it requires a migration script to update all existing subspaces to the new version. The script should take care of migrating all subspaces to have a default allowed fee tokens list.

[RiccardoM]

Is this actually true though? If we mark the Proto field as optional, do we need to migrate everything?

[dadamu]

Now proto3 fields are all optional now. You are right, adding new field does not cause the backward compatibility issues. I just learned from the official documentation. Thanks!

https://protobuf.dev/programming-guides/proto3/#updating
If you add new fields, any messages serialized by code using your “old” message format can still be parsed by your new generated code. You should keep in mind the default values for these elements so that new code can properly interact with messages generated by old code. Similarly, messages created by your new code can be parsed by your old code: old binaries simply ignore the new field when parsing. See the Unknown Fields section for details.