Refactor transaction flows to allow modifications of individual transactions

[0xivanov]

Description

Currently, the TokenRejectFlow class and similar transaction flows are overly reliant on class hierarchies that treat flows as if they are transactions themselves. This design complicates the implementation and maintenance of transaction flows.

To improve this, we should refactor transaction flows so that transactions are treated as members of the flows rather than flows inheriting from transactions. This approach will simplify the design by:

1. Avoiding the need to reimplement selectors, mutators, and other functionality whenever creating new flows.
2. Allowing individual transactions within a flow to be modified independently (e.g., setting specific execution or transaction parameters).

References

• Initial example improvement for TokenRejectFlow: feat: POC refactor for TokenRejectFlow hiero-sdk-cpp#868.