Add `must_use` attribute to `Mock`

[avandesa]

Description of changes:

It's easy to forget to mount a mock to a server, even though the docs call this out. This commit tags the Mock struct with #[must_use], so that Rust itself can warn users when they forget to mount their mocks. The help string

Here's the warning emitted:

warning: unused `Mock` that must be used
 --> examples\must_use.rs:9:5
  |
9 |     Mock::given(method("GET")).respond_with(response.clone());
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `Mock`s have to be mounted or registered with a `MockServer` to become effective
  = note: `#[warn(unused_must_use)]` on by default
help: use `let _ = ...` to ignore the resulting value
  |
9 |     let _ = Mock::given(method("GET")).respond_with(response.clone());
  |     +++++++

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[avandesa]

Guessing these are new lints, pretty sure they're not related to my change. Might be able to make a pr fixing them for you this evening if you want.

[LukeMathWalker]

That'd be nice if you have the time!