feat(middleware): add support for pipelines

[jbrockopp]

Dependent on #574 and #626

Part of the effort for go-vela/community#460

This adds support for the pipelines resource to the github.com/go-vela/server/router/middleware package.

This follows the exact same pattern and strategy for how we've added middleware support for other resources.

This will be used inside of #615 which adds API CRUD support for pipelines.

I've pulled this logic out into a separate PR to improve the experience when reviewing the code.

[codecov]

Codecov Report

Merging #627 (62cf15d) into master (d8f169f) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #627      +/-   ##
==========================================
+ Coverage   55.07%   55.22%   +0.15%     
==========================================
  Files         194      196       +2     
  Lines       15728    15781      +53     
==========================================
+ Hits         8662     8715      +53     
  Misses       6704     6704              
  Partials      362      362              

Impacted Files	Coverage Δ
router/middleware/pipeline/context.go	100.00% <100.00%> (ø)
router/middleware/pipeline/pipeline.go	100.00% <100.00%> (ø)

[cognifloyd]

I know this is WIP... Here's a quick comment from my early review.