chore: cleanup usage of share.Root(DAH)

[Wondertan]

Somewhere we were using Root and somewhere DAH. Also, we mostly use DAH generator from the app.
These PRs clean up all the usages and localize the Root/DAH type in share pkg as was originally intended.

Up to discussion is rename of share.Root to share.DAH to minimize discrepancies between app and node. Alternatively, if we wanna keep Root naming as it is, we should also rename DAH name on the ExtendedHeader(and son tags) and in other places, which is breaking.

Now, I think that calling it Root was not right and we should be calling the DataHash type Root, which is in fact the Merkle root of all the shares in the EDS.

[renaynay]

LGTM and thank you for this - are we completely sure it's not breaking? I think it still is bc of header field change despite being same underlying type.

[renaynay]

Units are also broken @Wondertan

[codecov-commenter]

Codecov Report

Merging #2481 (4423d2f) into main (7601c08) will decrease coverage by 0.10%.
The diff coverage is 52.17%.

@@            Coverage Diff             @@
##             main    #2481      +/-   ##
==========================================
- Coverage   51.35%   51.26%   -0.10%     
==========================================
  Files         158      159       +1     
  Lines       10666    10669       +3     
==========================================
- Hits         5478     5469       -9     
- Misses       4710     4721      +11     
- Partials      478      479       +1     

Files Changed	Coverage Δ
share/availability.go	0.00% <0.00%> (ø)
share/empty.go	0.00% <0.00%> (ø)
header/headertest/testing.go	81.30% <75.00%> (ø)
share/availability/cache/availability.go	82.50% <100.00%> (ø)
share/eds/eds.go	63.73% <100.00%> (ø)
share/eds/store.go	69.05% <100.00%> (ø)
share/getters/testing.go	83.33% <100.00%> (ø)

... and 3 files with indirect coverage changes

[walldiss]

Great cleanup PR!