add simple schema editor for output validation and encoding of numeric types

[mblackstock]

Currently all numbers are sent as float, and integers are encoded as a string, which is a bit odd. We need a way to specify the numeric types of fields in a more consistent and flexible way. This could be generalized to specify the type of each named field in a point for consistency.

One way to do this might be to have a schema editor in the output node that optionally specifies the types of fields in a message by name (boolean, int, uint, float, string). Under the hood this editor could use JSON schema in the configuration.

The output node could have a 'use schema' checkbox, and then you create a list of name/types. When a message is sent to the output node, it would ensure that the correct schema is used, and reject messages that it cannot convert. If a schema is not specified, a default encoding could be used - all numbers are float, strings, booleans sent as is.