Prototype Pollution in convict

This affects the package convict before 6.2.3. This is a bypass of CVE-2022-22143. The fix introduced, relies on the startsWith method and does not prevent the vulnerability: before splitting the path, it checks if it starts with proto or this.constructor.prototype. To bypass this check it's possible to prepend the dangerous paths with any string value followed by a dot, like for example foo.proto or foo.this.constructor.prototype.