fix: spec file validation now supports embedded structs

[fridrik01]

Context

We spend hours today debugging why we had long blocktimes in our devnet. After a lot of debugging we found out that the chain spec file used in our devnet had the wrong name for max block delay. This caused the node to start with MaxBlockDelay of 0seconds.

Fix
With the introduction of stable blocktime, we added an embedded struct to our chain spec file and current validation logic did not support embedded structs causing this issue to be missed on node startup. This PR addresses this.

Test plan

Rename max-block-delay to max-delay in our testing/files/spec.toml file. Then startup the node:

make start-custom testing/files/spec.toml

You should see an error like:

Error: failed to create chain spec: missing required configuration for key: block-delay-configuration.max-block-delay

[codecov]

Codecov Report

Attention: Patch coverage is 61.53846% with 15 lines in your changes missing coverage. Please review.

Project coverage is 60.79%. Comparing base (9d4ec3a) to head (ea96fcc).

Files with missing lines	Patch %	Lines
config/spec/creator.go	63.15%	11 Missing and 3 partials ⚠️
cli/commands/initialize/initialize.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                    @@
##           stable-block-time-pt2    #2830      +/-   ##
=========================================================
+ Coverage                  60.74%   60.79%   +0.04%     
=========================================================
  Files                        351      351              
  Lines                      16295    16328      +33     
=========================================================
+ Hits                        9898     9926      +28     
- Misses                      5631     5636       +5     
  Partials                     766      766              

Files with missing lines	Coverage Δ
cli/commands/initialize/initialize.go	54.81% <0.00%> (ø)
config/spec/creator.go	70.45% <63.15%> (+8.63%)	⬆️

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[calbera]

can we also add a test case where err is nil? i.e. the valid way to load block-delay-configuration, as reference

[calbera]

utACK