[TieredStorage] Add IndexOffset type

[yhchiang-sol]

Problem

TieredStorage conceptually has different offsets. However, the current code directly
uses the same primitive type for accessing offsets, which is error-prone as one could
easily use one offset to access data that should be accessed with a different offset
type.

Summary of Changes

This PR adds IndexOffset type -- a struct for obtaining the ith entry inside the
index-block to obtain account's offset and address.

[codecov]

Codecov Report

Merging #33929 (2770d74) into master (ec0ddc9) will decrease coverage by 0.1%.
The diff coverage is 75.0%.

@@            Coverage Diff            @@
##           master   #33929     +/-   ##
=========================================
- Coverage    81.9%    81.9%   -0.1%     
=========================================
  Files         811      811             
  Lines      219333   219337      +4     
=========================================
- Hits       179762   179712     -50     
- Misses      39571    39625     +54     

[yhchiang-sol]

Given this is a smaller PR, let me rebase it as we only have one comment here.

[brooksprumo]

lgtm