header: Extract `Head` method into separate `Head` interface, make `Syncer` implement it

[renaynay]

This PR extracts Head() method into a separate interface and makes Syncer implement it.

Prerequisite for #933 to restrict the State service's access to headers to the only methods it really needs (Head).

[codecov-commenter]

Codecov Report

Merging #978 (12aa560) into main (5859938) will increase coverage by 0.23%. The diff coverage is 81.81%.

@@            Coverage Diff             @@
##             main     #978      +/-   ##
==========================================
+ Coverage   58.56%   58.79%   +0.23%     
==========================================
  Files         131      132       +1     
  Lines        7766     7985     +219     
==========================================
+ Hits         4548     4695     +147     
- Misses       2745     2814      +69     
- Partials      473      476       +3     

Impacted Files	Coverage Δ
header/sync/sync.go	69.16% <77.77%> (+3.66%)	:arrow_up:
header/verify.go	82.97% <100.00%> (+1.58%)	:arrow_up:
node/services/service.go	80.66% <100.00%> (-2.79%)	:arrow_down:
node/settings.go	36.36% <100.00%> (+0.80%)	:arrow_up:
node/testing.go	100.00% <100.00%> (ø)
params/network.go	57.14% <100.00%> (+17.14%)	:arrow_up:
cmd/flags_misc.go	42.59% <0.00%> (-11.80%)	:arrow_down:
cmd/env.go	76.47% <0.00%> (ø)
... and 10 more

Help us with your feedback. Take ten seconds to tell us how you rate us.

[Wondertan]

Implementing the Head method for Syncer is the original goal of this PR. However, some internal logic has to be changed to allow this, and these logic changes are now extracted into https://github.com/celestiaorg/celestia-node/pull/990. This PR should rebase on top of it(after or before merge) and simply implement the Head method

[renaynay]

Already implemented in main