feat: add api-method-should-specify-api-operation rule

[unLomTrois]

I discovered in issue history that there was this rule once, but it checked ApiResponses, and was renamed

Due to need to cover the whole api with ApiOperations (#52), I present the actual rule for doing this.

[darraghoriordan]

hey, this looks ok. thanks for taking the time to add it.

can you update the readme also so folks know what it does?

thanks! dar

[darraghoriordan]

:tada: This PR is included in version 3.21.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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