chore(nodebuilder/das | pruner): Privatise samplingWindow in DAS params, add Duration method to AvailabilityWindow

[renaynay]

While this isn't really a bug fix, it's more of a "UX" fix - I'm removing SamplingWindow from the node's config so that it doesn't confuse users to see "0ms" as the config value even though in practice it's 30 days.

The reason there's a discrepancy is because we force the light sampling window in the DASer for light nodes in nodebuilder anyway so that config value is ignored regardless, meaning it does not need to be exposed.

Also adding Duration() method to return time.Duration instead of having to cast everywhere.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 44.73%. Comparing base (2469e7a) to head (088901f).
Report is 66 commits behind head on main.

Files	Patch %	Lines
pruner/window.go	50.00%	2 Missing ⚠️
nodebuilder/das/constructors.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3378      +/-   ##
==========================================
- Coverage   44.83%   44.73%   -0.10%     
==========================================
  Files         265      272       +7     
  Lines       14620    15271     +651     
==========================================
+ Hits         6555     6832     +277     
- Misses       7313     7657     +344     
- Partials      752      782      +30     

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[walldiss]

I have a couple of suggestions that could simplify the implementation:

1. Could we incorporate the default value for the SamplingWindow directly within the [DefaultParameters()](https://github.com/celestiaorg/celestia-node/blob/c04bbd251e7947180aadba793255e16e3126ef56/das/options.go#L56-L63) function? Could be just one-line addition.

2. Given that the SamplingWindow value is always overwritten in our current setup, exposing this setting to users seems unnecessary and potentially misleading. We should consider making SamplingWindow config field non-public. This approach would help prevent any confusion and ensure our configuration remains streamlined and intuitive.