new lint: missing_must_use_on_future_types

[rmehri01]

Adds a lint that checks if Future is implemented for a type (struct or enum) and will warn if there is no #[must_use] attribute since futures must be awaited to do work, similar to how async fn works.

Example output:

error: this struct implements `Future` but is missing a `#[must_use]` attribute
  --> tests/ui/missing_must_use_on_future_types.rs:11:1
   |
LL | struct BasicStruct;
   | ^^^^^^^^^^^^^^^^^^
   |
   = note: `-D clippy::missing-must-use-on-future-types` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::missing_must_use_on_future_types)]`
help: add the attribute
   |
LL + #[must_use]
LL | struct BasicStruct;
   |

Closes #13886

changelog: [missing_must_use_on_future_types]: new lint

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