feat: match authResponse username with 2.0 bns lookup

[hstove]

Fixes #953

This is the start of the work that will allow apps to handle an authResponse properly with 2.0 BNS.

After authentication, @stacks/auth calls verifyAuthResponse. This function runs a few checks, and also runs a check to ensure that the auth response is signed by the owner of the username provided in the payload.

Right now, you need to use a specific AppConfig to have a custom coreNodeUrl. When provided, it uses a different BNS lookup URL. For testing, I'm using our testnet registrar.

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[markmhendrickson]

@hstove should someone review these code changes now?

[reedrosenbluth]

LGTM! left one small style comment

[codecov]

Codecov Report

Merging #984 (4dd9e67) into master (dbab627) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #984      +/-   ##
==========================================
+ Coverage   60.69%   60.79%   +0.09%     
==========================================
  Files         101      101              
  Lines        7585     7596      +11     
  Branches     1412     1414       +2     
==========================================
+ Hits         4604     4618      +14     
+ Misses       2976     2973       -3     
  Partials        5        5              

Impacted Files	Coverage Δ
packages/auth/src/userSession.ts	57.74% <100.00%> (+0.60%)	⬆️
packages/auth/src/verification.ts	81.25% <100.00%> (+4.55%)	⬆️

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[andresgalante]

I this ready to merge?

[hstove]

Yes, ready for merge + release, cc @reedrosenbluth