chore: add LP mint calculation for stableswap pools

[kerber0x]

Description and Motivation

This PR fixes the LP mint calculation when providing liquidity for stableswap pairs.

Related Issues

---

Checklist:

• I have read Migaloo's contribution guidelines.
• My pull request has a sound title and description (not something vague like Update index.md)
• All existing and new tests are passing.
• I updated/added relevant documentation.
• The code is formatted properly cargo fmt --all --.
• Clippy doesn't report any issues cargo clippy -- -D warnings.
• I have regenerated the schemas if needed cargo schema.

[codecov]

Codecov Report

Attention: Patch coverage is 22.50000% with 62 lines in your changes missing coverage. Please review.

Project coverage is 92.98%. Comparing base (06cca15) to head (595ed83).

Files with missing lines	Patch %	Lines
...ity_hub/pool-network/terraswap_pair/src/helpers.rs	0.00%	45 Missing ⚠️
...ty_hub/pool-network/terraswap_pair/src/commands.rs	51.42%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #383      +/-   ##
==========================================
- Coverage   93.21%   92.98%   -0.23%     
==========================================
  Files         219      219              
  Lines       24867    24931      +64     
==========================================
+ Hits        23179    23182       +3     
- Misses       1688     1749      +61     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[0xFable]

Reviewed against the 3pool impl.
LGTM. The n_coins addition to compute_next_d is helpful for making it clear

[kerber0x]

Reviewed against the 3pool impl. LGTM. The n_coins addition to compute_next_d is helpful for making it clear

Yeah essentially removed the 3rd parameter from the 3pool and created the n_coins constant.