headertest: Consider unifying constructors for header generation and adding opts

[renaynay]

We currently have a few different ways to construct header test suite for generating headers whereas we could just unify them into a single constructor NewTestSuite and allow variadic amount of opts, like WithTimestampSpacing to set parameters on the test suite.

Lower priority, but would clean things up even further.

Related to #4032

[Hadijp99]

Good

[crStiv]

@renaynay i've made a PR, could you pls check it out when you'll have a free time

[pjdurden]

Hi @renaynay — I'd like to pick this up.

Plan: consolidate the overlapping RandExtendedHeader* / GenExtendedHeaders constructors in header/headertest/testing.go into a single functional-options API — e.g. RandExtendedHeader(t, opts ...HeaderOption) with options like WithHeight, WithChainID, WithTime, and WithValidatorSet. I'd keep thin backward-compatible wrappers where existing call sites depend on the current signatures so the migration stays mechanical and reviewable.

I noticed the earlier attempt in #4198 was closed unmerged — happy to align with whatever direction you'd prefer before I open the PR. Does this approach sound good?