add flash scope

[TomTriple]

This adds functionality for adding a message to the (cookie-based) flash scope and also a middleware for managing that scope automatically.

The message in the flash scope is kept for a single request only and is cleared afterwards by the provided middleware. At the moment one can only add a single string message to the flash scope. A more featureful implementation should provide support for a serialized key-value structure.

The implementation is still a bit rough (duplicate code ...), but it technically works as one would expect.

I could provide docs and support for key-value structure in a follow-up commit.

Adding middleware:

  override val run =
    Server.serve(appRoutes.toHttpApp @@ Middleware.flashScopeHandling).provide(Server.default)

Flash scope usage:

Response.seeOther("some url").addFlashMessage("overview not available at the moment")

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.05% ⚠️

Comparison is base (8780eb9) 64.21% compared to head (54ed9e9) 64.17%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2451      +/-   ##
==========================================
- Coverage   64.21%   64.17%   -0.05%     
==========================================
  Files         136      136              
  Lines        7118     7123       +5     
  Branches     1302     1245      -57     
==========================================
  Hits         4571     4571              
- Misses       2547     2552       +5     

Files Changed	Coverage Δ
zio-http/src/main/scala/zio/http/Middleware.scala	73.13% <0.00%> (-2.26%)	⬇️
zio-http/src/main/scala/zio/http/Request.scala	43.90% <0.00%> (-2.26%)	⬇️
zio-http/src/main/scala/zio/http/Response.scala	40.47% <0.00%> (-0.49%)	⬇️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.