Change OCEAN token to USDC

[trizin]

This pull request refactors the codebase to generalize the staking and payout token from a hardcoded "OCEAN" token to a hardcoded "USDC" token

CLI and Documentation Updates

• Renamed the payout claim CLI command and related code from claim_OCEAN to claim_payouts, and updated documentation and argument parsing to reflect the new naming.

Testing Refactor

• Updated all tests to use the USDC fixture and address, and ensured mocks and test logic are compatible with the new token abstraction.

Code Improvements

• Added a cached token_symbol property to FeedContract for improved logging clarity and reduced redundant contract calls.

These changes collectively make the codebase more flexible, maintainable, and ready for supporting USDC

[KatunaNorbert]

Doing a quick search in the code base I can see there are still multiple places where OCEAN token is mentioned, why aren't those updates as well?

[trizin]

@KatunaNorbert I updated it in a few other places.

I suggest we handle the rest of the changes in different PRs (readmes, code comments etc.), there are many and it's getting out of hand. I also added an item to the epic to update the faucet instructions.

Let's keep this PR focused on code changes that enable using USDC

[KatunaNorbert]

Alright, let's just make sure everything is updated as part of the epic

[trentmc]

What is the github issue for this? Please link to it in the PR title, and the PR description. (And that issue should link to the overall epic.) If these github issue(s) do not exist, then please create them.

[trentmc]

(I just renamed this PR to be in line with the naming in the code changes itself. Prediction token -> stake token.)