feat: Introduce the 'Assert' sink

[scarmuega]

With the goal of moving towards a CD pipeline, we introduce a new Assert sink.

The sink works by consuming incoming events and running an expanding set of "assertions" for each one. The result of the assertions are logged. If specified via configuration, a failed assertion will panic and terminate the pipeline (useful for automation).

In this initial PR we're including the following set of assertions:

block_depth_doesnt_skip_numbers
block_slot_increases
block_previous_hash_matches
event_timestamp_increases
tx_records_matches_block_count

Ideally, we'll keep growing the list with more specific checks for each event. There's also room for adding checks that include well-known, expected values (aka: asserting values for particular blocks).

The end-goal is to have a comprehensive set of assertions that can be used in automated integration tests, providing a fast mechanism for catching regressions on each new PR.

Implementation details:

• the sink maintains a state object which is the result of a "reduce" algorithm over the previous state and the current event. This is required since many of the interesting checks involve looking into the history of past events.
• we use macros to improve readability without sacrifying static dispatches.