Feat: Add AirSwap

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AirSwap (https://airswap.io/) is an RFQ DEX.
Counter-party discovery and token pricing are communicated off-chain; atomic token swaps are settled on-chain. AirSwap itself is a technology provider and runs no infrastructure of its own. More info on our wiki.

Notes
AirSwap uses a smart contract (Registry) for server URL discovery.

• We use StatefulEventSubscriber for the Registry.
• Registry is instantiated for every AirSwap instance.

AirSwap calls multiple third-party server URLs for pricing and orders.

• We use Fetcher to make HTTP calls on an interval.
• Fetcher is not instantiated for slave AirSwap instances.
• Prices are communicated to slaves via the cache.

Limitation: Tests cannot pass reliably on third-party servers, so:

• During tests an env var overrides URLs to point to a local server.
• Alternatively, we could pass a isLocal flag into initializePricing
• Alternatively, we could check process.env.NODE_ENV is test

Flow

1. initializePricing is called by the engine
   a. Registry starts up; hydrates; listens for event updates to track active server URLs
   b. Worker starts up (if not a slave process); writes pricing from active servers URLs to cache.
2. getPoolIdentifiers returns unique identifiers for active server URLs for a given token pair.
3. getPricesVolume pulls identifiers for a pair and reads the cache for pricing.
4. preProcessTransaction calls get*SideOrder on winning server.
5. getSimpleParam encodes the function call for the SwapERC20 contract.

Tests
For the initial review, simple tests are passing.
airswap-e2e.test.ts, airswap-events.test.ts, airswap-integration.test.ts.