[x/gamm][StableSwap] Add boilerplate for stableswap

[ValarDragon]

Adds boilerplate proto definitions for stableswap. This should not affect the current state machine

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[ValarDragon]

This function was duplicated, and existed in the types package.

[alexanderbez]

Curious, why the decision to have a sub-package called under x/gamm? Instead of:

• directly in x/gamm, OR
• separate module entirely

[alexanderbez]

nit: proto doc :p

[ValarDragon]

Not really sure what doc to put here yet, I think It'll become clearer when I go to make one go-side after an initial impl of the math. Added a placeholder for now though

[codecov-commenter]

Codecov Report

Merging #1214 (c76ace5) into main (eed3294) will decrease coverage by 0.04%.
The diff coverage is 28.12%.

@@            Coverage Diff             @@
##             main    #1214      +/-   ##
==========================================
- Coverage   21.03%   20.99%   -0.05%     
==========================================
  Files         196      196              
  Lines       25349    25345       -4     
==========================================
- Hits         5332     5320      -12     
- Misses      19054    19066      +12     
+ Partials      963      959       -4     

Impacted Files	Coverage Δ
x/gamm/keeper/pool.go	64.91% <0.00%> (ø)
x/gamm/pool-models/balancer/balancer_pool.go	62.06% <ø> (-1.82%)	⬇️
x/gamm/types/pool.go	0.00% <ø> (ø)
x/gamm/types/tx.pb.go	0.66% <ø> (ø)
x/gamm/keeper/pool_service.go	56.61% <23.07%> (-0.61%)	⬇️
x/gamm/keeper/invariants.go	29.41% <29.41%> (+29.41%)	⬆️
x/gamm/pool-models/balancer/msgs.go	81.08% <100.00%> (ø)

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[ValarDragon]

I'm down for changing how we do the proto package structure. I don't know whats standard.

Go-side, I think the x/gamm/pool-models/balancer and x/gamm/pool-models/stableswap; is better than x/gamm/balancer and x/gamm/stableswap. We eventually may have dozens here, and this packages them all into one pool-models directory.

Don't want to be in a situation where we have 10+ pool models clogging the top level go structure, and they each have disparate names that makes the folder structure not as quickly grokable.

[mattverse]

LGTM!

I also agree with @ValarDragon that we maintain the sub-package structure where different gamm models are structured like x/gamm/pool-models/balancer and x/gamm/pool-models/stableswap. One of the reasons I'm in support of this structure is that go import cycle can be rather tricky and keeping a sub-package is helpful in managing this import cycle structure between packages.

[ValarDragon]

Lets discuss package structure in a separate issue, going to go ahead and merge this to simplify rebasing further logic work on stableswap!