feat: add metadata field to Ticker and MarketConfig

[aljo242]

Closes BLO-913

Adds metadata fields to msg index 15 for both messages. This gives room for the messages to be extended if necessary, while using a field index that still takes one byte to encode.

Add IsValid() function to new pkg/json package. This checks the basic validity of if a byte array is json.

The metadata fields are chosen as string and not bytes in the proto, because strings can be compared as map keys.

[aljo242]

One other thing to note is that we probably should choose some length limit for the JSON strings. I'm not sure what that limit would look like and would like to discuss this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a904d5a) 74.96% compared to head (b3ee976) 74.99%.

Additional details and impacted files

@@                Coverage Diff                 @@
##           feat/marketmap     #128      +/-   ##
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+ Coverage           74.96%   74.99%   +0.02%     
==================================================
  Files                 147      148       +1     
  Lines                5959     5966       +7     
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+ Hits                 4467     4474       +7     
  Misses               1123     1123              
  Partials              369      369              

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[Eric-Warehime]

LGTM. In terms of adding limits to the json size...since it's a string I guess it's slightly harder to reason about encoding size? Maybe just saying 1K and seeing if we ever need to break that is reasonable. Since the updates are currently constrained by the permissioned actor and in the future might have some permissionless element to them, I don't think not validating this poses and huge risk.