feat(crosschain): add rate limiter flags singleton state variable

[lumtis]

Description

This self contained object will contains all information for rate limiting

• Add message UpdateRateLimiterFlags
• Add query
• Add variable to genesis

message RateLimiterFlags {
  bool enabled = 1;

  // window in blocks
  int64 window = 2;

  // rate in azeta per block
  string rate = 3 [
    (gogoproto.customtype) = "github.com/cosmos/cosmos-sdk/types.Uint",
    (gogoproto.nullable) = false
  ];

  // conversion in azeta per token
  repeated Conversion conversions = 4 [(gogoproto.nullable) = false];
}

message Conversion {
  string zrc20 = 1;
  string rate = 2 [
    (gogoproto.customtype) = "github.com/cosmos/cosmos-sdk/types.Dec",
    (gogoproto.nullable) = false
  ];
}

Closes: #2041

[github-actions]

!!!WARNING!!!
nosec detected in the following files: x/crosschain/types/keys.go

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.25%. Comparing base (583bfaf) to head (ac8590d).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2046      +/-   ##
===========================================
+ Coverage    71.11%   71.25%   +0.14%     
===========================================
  Files          245      250       +5     
  Lines        13643    13712      +69     
===========================================
+ Hits          9702     9771      +69     
  Misses        3495     3495              
  Partials       446      446              

Files	Coverage Δ
x/crosschain/genesis.go	100.00% <100.00%> (ø)
...crosschain/keeper/grpc_query_rate_limiter_flags.go	100.00% <100.00%> (ø)
...ain/keeper/msg_server_update_rate_limiter_flags.go	100.00% <100.00%> (ø)
x/crosschain/keeper/rate_limiter_flags.go	100.00% <100.00%> (ø)
x/crosschain/types/genesis.go	77.41% <100.00%> (ø)
...sschain/types/message_update_rate_limiter_flags.go	100.00% <100.00%> (ø)
x/crosschain/types/rate_limiter_flags.go	100.00% <100.00%> (ø)

[skosito]

lgtm, only minor comments