Add basic firestore implementation

theodolite-benchmarks/uc1-beam/build.gradle

...marks/uc1-beam/src/main/java/application/GenericSink.java

...marks/uc1-beam/src/main/java/application/SinkFactory.java

...nchmarks/uc1-beam/src/main/java/application/SinkType.java

...s/uc1-beam/src/main/java/application/Uc1BeamPipeline.java

...m/src/main/java/application/firestore/DocumentMapper.java

.../src/main/java/application/firestore/FirestoreConfig.java

...am/src/main/java/application/firestore/FirestoreSink.java

...ain/java/application/firestore/UpdateOperationMapper.java