Implement topology-aware validations

[lukesteensen]

In contrast with validations that check the configuration of a single source/transform/sink, we should implement validations that look at the topology as a whole.

Implementation-wise, this is going to look a lot like a type checker. Sources are equivalent to fn() -> T, transforms to fn(T) -> U, and sinks to fn(U). The specific types will be different based on how each is configured (i.e. parsers, encoders, etc). Ideally, such a system would allow us to easily catch cases like the following:

• Sending log data into a sink that expects metrics
• Sending structured data to a raw sink with no encoder configured
• Placing a sampler configured to read a specific field after a raw source that can't possibly set that field
• Placing a parser configured to read a record's original raw data after a source that only provides structured fields

There is potentially a lot of complexity here, so we should probably dedicate some time for additional research, a spike implementation, and a more detailed proposal. There's some temptation to tie these "types" to the variants of our Record/event type (which is reasonable), but it could also be interesting to explore a more field-oriented direction (i.e. structural instead of nominative typing).