move index storage into interface

[bobcallaway]

this opens up the opportunity to use other storage mediums for indices

[codecov]

Codecov Report

Merging #1741 (797bdaf) into main (75774ff) will increase coverage by 0.11%.
The diff coverage is 82.00%.

@@            Coverage Diff             @@
##             main    #1741      +/-   ##
==========================================
+ Coverage   66.76%   66.88%   +0.11%     
==========================================
  Files          88       90       +2     
  Lines        8919     8951      +32     
==========================================
+ Hits         5955     5987      +32     
  Misses       2252     2252              
  Partials      712      712              

Flag	Coverage Δ
e2etests	48.58% <64.00%> (+0.04%)	⬆️
unittests	47.73% <30.95%> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
cmd/rekor-server/app/root.go	61.38% <100.00%> (+0.78%)	⬆️
pkg/api/error.go	67.39% <ø> (ø)
pkg/indexstorage/redis/redis.go	100.00% <100.00%> (ø)
pkg/api/entries.go	64.75% <66.66%> (ø)
pkg/indexstorage/indexstorage.go	66.66% <66.66%> (ø)
pkg/api/api.go	68.96% <70.00%> (-1.84%)	⬇️
pkg/api/index.go	69.56% <62.50%> (ø)

... and 2 files with indirect coverage changes