Adds dappId to combinator contracts

[acenolaza]

Closes #24

Changes

• Adds dappId immutable variable to all combinator contracts. These are set by reading the underlying proxy.dappId() (except for NormalizedApi3ReaderProxyV1)
• Casts underlying proxies to IApi3ReaderProxyV1
• NormalizedApi3ReaderProxyV1 now requires a dappId to be pass in the constructor
• Validates that proxies dappId are equal in ProductApi3ReaderProxyV1
• Replaces @api3/contracts imports with MockApi3ReaderProxyV1 test contract to make testing simpler

Notes

• The dappId immutable variable is needed because dappId() in IApi3ReaderProxyV1 is not marked as view
• The cast to IApi3ReaderProxyV1 still feels kind of wrong to me since the underlying proxies might be another combinator proxy which might not even expose a dapiName() function for example. I think it might be more accurate to create a IApi3ReaderProxyWithDappId or similar that inherints from IApi3ReaderProxyand adds a function dappId() external view returns (uint256);