thermal-service: Add mocks

[kurtjd]

Adds mock sensor and fan to thermal service and updates the thermal std example to make use of them (which simplifies the example quite a lot).

Example can be tested with cargo run --release --bin thermal.

Resolves #619.

[jerrysxie]

Good start, for future expansion, we can consider adding data modeling to the mock so the mock can take a data model to generate different test cases.

[tullom]

Good start, for future expansion, we can consider adding data modeling to the mock so the mock can take a data model to generate different test cases.

I've been thinking of a trait that can encapsulate this type of behaviour and also allow for easy fuzz testing. Will create an issue for this. Created #712 to track