Maintenance: Make `@middy/core` a `devDependency`

[dreamorosi]

Description of the feature request

Problem statement

Middy (@middy/core) is currently used in the middlewares implementations only for types (as brought up by @saragerion here).

We should move it as devDependency to avoid bundling it to Customers who might not be using it.

• Move @middy/core to devDependencies for all packages
• Update notices in utilities docs to specify that customers who want to use the middleware must install it

Summary of the feature

N/A

Code examples

N/A

Benefits for you and the wider AWS community

Avoid including unnecessary dependencies for those who don't use middy middlewares.

Describe alternatives you've considered

N/A

Additional context

N/A

Related issues, RFCs

#370

[michaelbrewer]

Thanks @dreamorosi for adding this.

[github-actions]

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