OR-5253 TransformingOperators:: flatMap

[crazymanish]

Context

• Close ticket: [w20(14may-20may)] Operators:: Transforming Operators: Flattening publishers #16

Operators are publishers

• In Combine, methods that perform an operation on values coming from a publisher are called operators.
• Each Combine operator actually returns a publisher. Generally speaking, that publisher receives the upstream values, manipulates the data, and then sends that data downstream.

In this PR

• TransformingOperators:: flatMap
• flatMap(maxPublishers:_:) Transforms all elements from an upstream publisher into a new publisher up to a maximum number of publishers you specify.
• https://developer.apple.com/documentation/combine/publishers/collect/flatmap(maxpublishers:_:)-36djm