feat(earn): Add feature gate for covering gas fees

[finnian0826]

Description

feature gate: https://console.statsig.com/4plizaPmWwPL21ASV4QAO0/gates/subsidize_stablecoin_earn_gas_fees

Test plan

N/A

Related issues

• Fixes #ACT-1190

Backwards compatibility

Yes

Network scalability

If a new NetworkId and/or Network are added in the future, the changes in this PR will:

• Continue to work without code changes, OR trigger a compilation error (guaranteeing we find it when a new network is added)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.33%. Comparing base (6c3366d) to head (fc4ba52).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5437      +/-   ##
==========================================
+ Coverage   86.32%   86.33%   +0.01%     
==========================================
  Files         755      755              
  Lines       30887    30943      +56     
  Branches     5276     5281       +5     
==========================================
+ Hits        26664    26716      +52     
- Misses       3996     4000       +4     
  Partials      227      227              

Files	Coverage Δ
src/statsig/constants.ts	100.00% <ø> (ø)
src/statsig/types.ts	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[silasbw]

nit: SUBSIDIZE instead of COVER? (it seems like that's the term we've been using 🤷 )