refactor(database): move worker logic into separate package

[jbrockopp]

Based off of https://github.com/go-vela/server/pull/574, https://github.com/go-vela/server/pull/663 and https://github.com/go-vela/server/pull/687

This change continues the refactor efforts initially introduced in the above PRs.

This adds a new worker package to the github.com/go-vela/server/database package.

This contains a WorkerService interface declaring all functions necessary for worker based interactions with the database:

https://github.com/go-vela/server/blob/7991f03903622a820698678037cbe482828d72a2/database/worker/service.go#L11-L43

This package also contains the engine which implements the above service interface:

https://github.com/go-vela/server/blob/7991f03903622a820698678037cbe482828d72a2/database/worker/worker.go#L23-L37

This engine contains no raw SQL queries for integrating with the workers table.

Instead, we leverage our DB library's (https://gorm.io/) agnostic abstraction for integrating with that table.

[codecov[bot]]

Codecov Report

Merging #692 (729481d) into main (4335e56) will decrease coverage by 0.14%. The diff coverage is 77.46%.

@@            Coverage Diff             @@
##             main     #692      +/-   ##
==========================================
- Coverage   54.94%   54.79%   -0.15%     
==========================================
  Files         211      216       +5     
  Lines       16030    16015      -15     
==========================================
- Hits         8808     8776      -32     
- Misses       6845     6857      +12     
- Partials      377      382       +5     

Impacted Files	Coverage Δ
api/build.go	1.64% <0.00%> (ø)
api/metrics.go	0.00% <0.00%> (ø)
api/worker.go	0.00% <0.00%> (ø)
router/middleware/executors/executors.go	3.70% <0.00%> (ø)
database/worker/worker.go	51.72% <51.72%> (ø)
database/worker/list.go	72.72% <60.86%> (ø)
database/sqlite/sqlite.go	63.44% <62.50%> (+1.26%)	:arrow_up:
database/postgres/postgres.go	63.56% <70.00%> (+1.52%)	:arrow_up:
database/worker/get.go	81.25% <81.25%> (ø)
database/worker/get_hostname.go	83.33% <83.33%> (ø)
... and 9 more

[jbrockopp]

@cognifloyd GetWorkerByAddress was not used anywhere in the codebase 😅

When I discovered this during the refactor, I elected to not include it.

However, if we feel there is a good reason to keep it, I could certainly do so.