[oracle] add timestamp to oracle exchange rates

[udpatil]

Describe your changes and provide context

This adds last_update_timestamp to the data stored for exchange rates. This allows for calculating stalesness of oracle prices based on timestamp for use by smart contracts.

For this change, no migration is necessary because we don't have the tiemstamps of previously updated oracle prices available, and the timestamp will only be present as 0 for one oracle window prior to the next oracle update. There won't yet be contracts reading that values until after the timestamps have been established.

Testing performed to validate your change

Updated unit tests + verified compatibility for existing cosmwasm version + verified compatibility with updated sei-cosmwasm struct to have the timestamp field.

[codecov]

Codecov Report

Merging #1073 (2744f68) into main (4fdcf14) will increase coverage by 0.22%.
The diff coverage is 69.23%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1073      +/-   ##
==========================================
+ Coverage   63.41%   63.64%   +0.22%     
==========================================
  Files         256      256              
  Lines       16593    16597       +4     
==========================================
+ Hits        10523    10563      +40     
+ Misses       5567     5530      -37     
- Partials      503      504       +1     

Files	Coverage Δ
x/oracle/keeper/keeper.go	94.58% <100.00%> (+0.01%)	⬆️
x/oracle/keeper/querier.go	58.76% <100.00%> (+0.86%)	⬆️
x/oracle/types/vote.go	35.08% <0.00%> (-0.63%)	⬇️

... and 3 files with indirect coverage changes