Lightweight event framework for microservices

[peterbroadhurst]

The FFTM codebase has built-in eventing support with:

• WebSockets support for inbound connections
• Webhooks support for outbound connections
• Workload managed mode: at-least-once delivery
• Broadcast mode: at-most-once delivery
• Batching for performance
• Checkpointing for the at-least-once delivery assurance
• CRUD on event streams, using DB backed storage

This PR adapts that framework to be generic, such that it can be embedded into any Golang microservice.

The source of the events can be anything with the following characteristics:

• Events have a sequence identifier, that can be expressed as an alpha-numerically sortable string
• Events can be replayed from a specific previous sequence ID

Some improvements in code structure have been made along the way, in making it generic.
Also E2E tests have been added that demonstrate how it could be embedded, and using a real DB + WebSockets/WebHooks delivery mechanism.

[codecov-commenter]

Codecov Report

Merging #105 (c5acf45) into main (6f614c3) will decrease coverage by 0.02%.
The diff coverage is 99.90%.

@@             Coverage Diff             @@
##              main     #105      +/-   ##
===========================================
- Coverage   100.00%   99.98%   -0.02%     
===========================================
  Files           63       77      +14     
  Lines         5048     6100    +1052     
===========================================
+ Hits          5048     6099    +1051     
- Misses           0        1       +1     

Files	Coverage Δ
pkg/dbsql/database.go	100.00% <100.00%> (ø)
pkg/dbsql/provider.go	100.00% <ø> (ø)
pkg/dbsql/provider_sqlitego.go	100.00% <100.00%> (ø)
pkg/eventstreams/activestream.go	100.00% <100.00%> (ø)
pkg/eventstreams/config.go	100.00% <100.00%> (ø)
pkg/eventstreams/event.go	100.00% <100.00%> (ø)
pkg/eventstreams/eventstreams.go	100.00% <100.00%> (ø)
pkg/eventstreams/manager.go	100.00% <100.00%> (ø)
pkg/eventstreams/persistence.go	100.00% <100.00%> (ø)
pkg/eventstreams/webhooks.go	100.00% <100.00%> (ø)
... and 14 more