move generics from chain-gateway to near-contract-transport

[kevindeforth]

Background

In #4266 we generalized and cleaned some traits from the chain-gateway crate.
Those traits and types are now generic enough to be useful for a multitude of back-ends.
We should move them to the near-contract-transport crate, sch that other back-ends can subscribe to contract state without needing to import the entire chain-gateway crate.

User Story

As a dev, I like order in the code-base, I like small-scoped easy to review PRs and I like to re-use components when useful.

Acceptance Criteria

Move the following to the near-contract-transport crate:

• state_viewer/monitoring.rs
• state_viewer/subscripiton.rs
• state_vewer/traits.rs
• state_vewer/view_call.rs
• the MockViewer from mock.rs

Resources & Additional Notes

No response