feat: add mm portfolio api with env based feature flag

[Bigshmow]

Description

1. What is the reason for the change?
   To provide portfolio api access to MM mobile in order to begin token search work
   Adding "feature flag"
2. What is the improvement/solution?
   Setup our basic fetch functionality, including a health check
   Flag is added in line with this previously merged PR

Pre-merge author checklist

• I’ve followed MetaMask Contributor Docs and MetaMask Mobile Coding Standards.
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

[github-actions]

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❌ @devin Stewart

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[sonarqubecloud]

Quality Gate failed

Failed conditions
16.7% Coverage on New Code (required ≥ 60%)

See analysis details on SonarQube Cloud