chore: move share encoding logic from core -> app

[evan-forbes]

Description

This PR moves the share encoding code from core to the app. It makes as few changes as possible, as to make the upcoming refactor for non-interactive defaults actually reviewable.

the code originally came from:
https://github.com/celestiaorg/celestia-core/blob/5c844685421fcb0ff328bba1752d080420002f41/types/shares.go
https://github.com/celestiaorg/celestia-core/blob/5c844685421fcb0ff328bba1752d080420002f41/types/shares_test.go
https://github.com/celestiaorg/celestia-core/blob/5c844685421fcb0ff328bba1752d080420002f41/types/share_splitting.go
https://github.com/celestiaorg/celestia-core/blob/5c844685421fcb0ff328bba1752d080420002f41/types/share_merging.go

closes: #623

[codecov-commenter]

Codecov Report

Merging #627 (8a25644) into main (24bbf82) will increase coverage by 8.75%.
The diff coverage is 75.10%.

❗ Current head 8a25644 differs from pull request most recent head 421f233. Consider uploading reports for the commit 421f233 to get more accurate results

@@            Coverage Diff             @@
##             main     #627      +/-   ##
==========================================
+ Coverage   29.78%   38.53%   +8.75%     
==========================================
  Files          16       19       +3     
  Lines        2055     2543     +488     
==========================================
+ Hits          612      980     +368     
- Misses       1382     1478      +96     
- Partials       61       85      +24     

Impacted Files	Coverage Δ
pkg/shares/shares.go	50.00% <50.00%> (ø)
pkg/shares/share_splitting.go	75.00% <75.00%> (ø)
pkg/shares/share_merging.go	77.11% <77.11%> (ø)
x/payment/types/payfordata.go	77.30% <100.00%> (+1.95%)	⬆️

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[adlerjohn]

utACK. Can you add to the PR description a permalink to the source of the code?

[rach-id]

utAck