Add a way to update the `admin` principal in subnet.clar

[codecov[bot]]

Codecov Report

Merging #294 (c422eb0) into develop (06d6ea8) will increase coverage by 0.11%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #294      +/-   ##
===========================================
+ Coverage    86.27%   86.38%   +0.11%     
===========================================
  Files            6        6              
  Lines          357      360       +3     
  Branches        72       73       +1     
===========================================
+ Hits           308      311       +3     
  Misses          49       49              

Impacted Files	Coverage Δ
core-contracts/contracts/subnet.clar	91.25% <100.00%> (+0.10%)	:arrow_up:

[blockstack-devops]

:tada: This PR is included in version 0.7.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: