Soften extension key pattern in TokenCreationValidator

Currently the validator uses the strict patter /^((?!(tx|user|pages|tt|sys|ts_language|csh))[a-z]{1}[a-z0-9_]+)$/ which is used for creating new extension keys. However, since there are already extensions (like tt_address) which do not follow this pattern, this must be replaced with a less strict one (something like /^[a-z][a-z0-9_]{2,29}$/). Otherwise access tokens can not be restricted to such extension keys.

As a side task, the pattern used in the OpenAPI specification for the key parameter should also be changed to this pattern since it currently contains a meaningless quantifier.