Implements ReadableAccount for RefMut Account and AccountSharedData

[brooksprumo]

Problem

From this comment, https://github.com/solana-labs/solana/pull/34194/files#r1420665797, we'd like to use RefMut<AccountSharedData> as a ReadableAccount, but cannot directly. There are already implementations for Ref<_>, so this is just a gap.

Summary of Changes

Implements ReadableAccount for RefMut Account and AccountSharedData.

[codecov]

Codecov Report

Merging #34370 (268ac77) into master (5c00a33) will increase coverage by 0.0%.
Report is 22 commits behind head on master.
The diff coverage is 38.4%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #34370   +/-   ##
=======================================
  Coverage    81.9%    81.9%           
=======================================
  Files         819      819           
  Lines      220422   220461   +39     
=======================================
+ Hits       180652   180685   +33     
- Misses      39770    39776    +6     

[brooksprumo]

Closing. From #34194:

Since RefMut implements Deref<T>, callers can do a .deref() (or *) instead.