Add a "not implemented" stage to maturity levels

[jpkrohling]

Let's take profiling as an example - if it becomes a new signal with a new procotol or data model, then it would be added to the specification first. Once it's in the specification, it would be added to the collector thus initially being not implemented.

In either case, not implemented should likely be an optional state - client libraries definitely have this today.

Originally posted by @flands in #232 (comment)

[svrnm]

+1 for this, it's also already used on the website for signals not implemented in certain APIs/SDKs: https://opentelemetry.io/docs/instrumentation/

[carlosalberto]

Defined a P1 priority initially (from the maintainers call). @jpkrohling any opinion?

[jpkrohling]

@flands, are you interested in sending a PR with this change? We discussed something like this in the past, where such inclusions would be done in two stages: first, the new signal is optional, and a grace period would be given to SDKs to incorporate the new signal. After this grace period, it becomes a requirement to declare a component as 'stable'.