AsyncAPI 2.x.y - validation of runtime expression

[char0n]

We are currently not able to validate runtime expression. Ideally using already existing tool is ideal, but if we cannot find one we can still write one. I have created the parser and validator library here: https://github.com/char0n/asyncapi-runtime-expression

Implementation will apply for AsyncAPI 2.x.y and possibly also for AsyncAPI 1.x.y.

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The runtime expression is defined by the following ABNF syntax.

expression = ( "$message" "." source )
source = ( header-reference | payload-reference )
header-reference = "header" ["#" fragment]
payload-reference = "payload" ["#" fragment]
fragment = a JSON Pointer [RFC 6901](https://tools.ietf.org/html/rfc6901)