feat: add pretty printed default to validated flags

[pmalek]

What this PR does / why we need it:

This adds a pretty printed default to ValidatedValueOpt[T] so that when one uses it as such:

flagSet.Var(flags.NewValidatedValue(&c.GatewayAPIControllerName, gatewayAPIControllerNameFromFlagValue, flags.WithDefault(string(gateway.GetControllerName()))), "gateway-api-controller-name", "The controller name to match on Gateway API resources.")

Instead of

--gateway-api-controller-name string             The controller name to match on Gateway API resources.

the end user can get

--gateway-api-controller-name string             The controller name to match on Gateway API resources. (default "konghq.com/kic-gateway-controller")

This will be useful for e.g. custom types like DNSStrategy in #4071.

Additionally this PR moves the types and functions related to validated flags to a separate package under internal/manager/flags.

[codecov]

Codecov Report

Patch coverage: 81.2% and project coverage change: -38.4 ⚠️

Comparison is base (ccafa7c) 59.9% compared to head (1e1d48d) 21.5%.

❗ Current head 1e1d48d differs from pull request most recent head 5f98b2d. Consider uploading reports for the commit 5f98b2d to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #4078      +/-   ##
========================================
- Coverage   59.9%   21.5%   -38.4%     
========================================
  Files        149     149              
  Lines      16520   16532      +12     
========================================
- Hits        9899    3567    -6332     
- Misses      5992   12491    +6499     
+ Partials     629     474     -155     

Impacted Files	Coverage Δ
internal/manager/flags/validated.go	70.4% <75.0%> (ø)
internal/manager/config.go	94.5% <100.0%> (ø)

... and 116 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.