style(cqrs): fix typing for CommandHandler to actually accept the type of class extending ICommand

[NOtherDev]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe: Typing fix

What is the current behavior?

I extended ICommand interface via TypeScript's Declaration Merging, adding own properties there, so that I can enforce all my commands to adhere to some standard format. This made my code not compile due to invalid/incomplete typing for CommandHandler decorator, which expects ICommand instance passed as an argument, while the documented intention is that it accepts ICommand-compatible type.

Issue Number: N/A

What is the new behavior?

I extended the existing typing so that the actual class type that implements ICommand is accepted by CommandHandler decorator.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

lgtm