Allow withdrawer to change the authorized stake key

[garious]

Problem

Users are likely to treat their "authorized staker" key as a hot key and keep their "authorized withdrawer" key in cold storage. If the staker key is compromised and set to a new key by an attacker, there's currently no way to use a "master" key from cold storage to reset it.

Summary of Changes

Permit the withdrawer key to change the staker key.

Note: This does not make the same change to the Vote program for two reasons:

1. It feels less important. The stake accounts typically have more tokens than vote accounts.
2. The Vote program has additional logic related to which epoch the was key changed in, making the patch complex enough that I got intimidated and backed out.

Fixes #8446

[codecov]

Codecov Report

Merging #8456 into master will increase coverage by <.1%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #8456     +/-   ##
========================================
+ Coverage    80.3%   80.3%   +<.1%     
========================================
  Files         254     254             
  Lines       56235   56270     +35     
========================================
+ Hits        45173   45209     +36     
+ Misses      11062   11061      -1

[aeyakovenko]

seems ok to me