feat: introduce DefaultMinGasPrice to appconsts

[cmwaters]

Uses a constant in th appconsts package for the DefaultMinGasPrice that users can import.

[evan-forbes]

Nice!

[codecov-commenter]

Codecov Report

Merging #1642 (fe6c6bf) into main (11374a6) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1642      +/-   ##
==========================================
+ Coverage   50.89%   50.96%   +0.06%     
==========================================
  Files          92       92              
  Lines        5739     5751      +12     
==========================================
+ Hits         2921     2931      +10     
- Misses       2518     2520       +2     
  Partials      300      300              

Impacted Files	Coverage Δ
testing/txsim/send.go	89.65% <ø> (ø)
app/write_square.go	92.98% <100.00%> (ø)
pkg/inclusion/paths.go	100.00% <100.00%> (ø)
pkg/shares/non_interactive_defaults.go	100.00% <100.00%> (ø)
pkg/shares/share_splitting.go	61.73% <100.00%> (ø)
testing/txsim/account.go	67.44% <100.00%> (ø)
x/blob/types/payforblob.go	71.42% <100.00%> (-0.42%)	⬇️

... and 1 file with indirect coverage changes

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