Add a new snapshot directory config

[yzang2019]

Describe your changes and provide context

This change add a new config snapshot-directory in app.toml to override default directory for storing state sync snapshot.

Testing performed to validate your change

Tests are covered in sei-protocol/sei-chain#944

[codecov]

Codecov Report

Merging #305 (dc21be6) into main (58083f6) will decrease coverage by 0.02%.
The diff coverage is 6.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #305      +/-   ##
==========================================
- Coverage   55.37%   55.36%   -0.02%     
==========================================
  Files         620      620              
  Lines       51599    51611      +12     
==========================================
+ Hits        28571    28572       +1     
- Misses      20947    20958      +11     
  Partials     2081     2081              

Impacted Files	Coverage Δ
baseapp/baseapp.go	72.32% <ø> (ø)
baseapp/options.go	54.36% <0.00%> (-2.29%)	⬇️
server/config/toml.go	17.85% <ø> (ø)
server/start.go	21.54% <ø> (ø)
simapp/simd/cmd/root.go	68.00% <0.00%> (-1.11%)	⬇️
server/config/config.go	37.08% <50.00%> (+0.17%)	⬆️