Add FXMacroData client

[roberttidball]

Summary

• add a Java 8-compatible FXMacroData client using the repo's existing Apache HttpClient, Jackson, and RxJava stack
• cover catalogue, announcements, predictions, forex, COT, commodities, curves, differentials, sessions, risk sentiment, news, press releases, generic requests, and GraphQL
• add JUnit tests for URL construction across the endpoint surface

Validation

• mvn -Dtest=io.codera.quant.data.FxMacroDataClientTest test is blocked by the repo's existing unresolved tws-api:tws-api:9.72.17-SNAPSHOT dependency
• direct compile/test check passed with public Maven dependencies: javac -source 8 -target 8 ... FxMacroDataClient.java FxMacroDataClientTest.java and java org.junit.runner.JUnitCore io.codera.quant.data.FxMacroDataClientTest