Add account min balance field

[jasonpaulos]

Summary

Add the minimum balance field to accounts returned over the REST API.

Fixes #808

Test Plan

Added a minBalance unit test exercising the core minBalance formula (parallels that in go-algorand).

[codecov]

Codecov Report

Attention: Patch coverage is 93.05556% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 68.80%. Comparing base (ef8395b) to head (7c1a5a3).
Report is 1 commits behind head on main.

Files	Patch %	Lines
idb/postgres/postgres.go	58.33%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1596      +/-   ##
==========================================
+ Coverage   68.54%   68.80%   +0.25%     
==========================================
  Files          37       38       +1     
  Lines        7452     7517      +65     
==========================================
+ Hits         5108     5172      +64     
- Misses       1914     1915       +1     
  Partials      430      430              

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[jasonpaulos]

✅ Looks good to me. I cannot officially approve since I opened this PR

[cce]

LGTM