feat: support EIP-7702 gasless swaps via Relay /execute endpoint

[matthewwalsh0]

Explanation

On EIP-7702 supported chains, the Relay strategy now submits source transactions via Relay's /execute endpoint instead of adding them through the TransactionController. This enables gasless execution where Relay's relayer submits the transaction on behalf of the user.

• Source calls (approve + deposit) are combined into a single delegation transaction using getDelegationTransaction
• The combined transaction is submitted to Relay's /execute endpoint, which handles gas payment
• Non-EIP-7702 chains continue using the existing addTransaction/addTransactionBatch flow
• All Relay network interactions are encapsulated in a new relay-api.ts module
• getEIP7702SupportedChains replaced with isEIP7702Chain(messenger, chainId) for cleaner call sites
• Execute and quote URLs are configurable via feature flags for proxy support

References

• Relay gasless swaps documentation
• Relay /execute API reference

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them