feat(earn): Add feature gate for covering gas fees (#5437)

### 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:

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

src/statsig/constants.ts

@@ -28,6 +28,7 @@ export const FeatureGates = {
   [StatsigFeatureGates.SHOW_JUMPSTART_SEND]: false,
   [StatsigFeatureGates.SHOW_POINTS]: false,
   [StatsigFeatureGates.SHOW_STABLECOIN_EARN]: false,
+  [StatsigFeatureGates.SUBSIDIZE_STABLECOIN_EARN_GAS_FEES]: false,
 } satisfies { [key in StatsigFeatureGates]: boolean }
 
 export const ExperimentConfigs = {

src/statsig/types.ts

@@ -31,6 +31,7 @@ export enum StatsigFeatureGates {
   SHOW_JUMPSTART_SEND = 'show_jumpstart_send',
   SHOW_POINTS = 'show_points',
   SHOW_STABLECOIN_EARN = 'show_stablecoin_earn',
+  SUBSIDIZE_STABLECOIN_EARN_GAS_FEES = 'subsidize_stablecoin_earn_gas_fees',
 }
 
 export enum StatsigExperiments {