Create the rule asyncapi3-message-examples for the v3 core ruleset

[smoya]

Part of #1016 (included in Bounty).

Add the following rule for the v3 core ruleset:

Based on asyncapi2-message-examples: Examples of message object should validate againt the "payload" and "headers" schemas.

[github-actions]

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[TusharMohapatra07]

@smoya Is this issue still open for taking PRs ?