[EVM] DRAFT - Replace EVM.BridgeRouter interface with implementation

[sisyphusSmiling]

Related: onflow/flow-evm-bridge#60

While redeploying bridge contracts to Previewnet, I realized that the Accessor-Router pattern originally introduced is subverted by the Router being defined in a bridge contract. The idea originally was to define a neutral party router which would store a bridge-defined accessor through which the EVM contract would pass bridge calls to from COAs. However, currently, the BridgeRouter implementation is defined in a bridge contract which, in the event of a redeployment, must necessarily be replaced thus defeating the original purpose of the design.

This draft PR seeks to address this issue by replacing the interface with a resource definition within the EVM contract. I'm leaving this as a draft to serve as a point of discussion for a couple of points I could use clarity on:

1. Removing an interface declaration is a breaking change - is this allowable on PreviewNet?
   a. If not, we could leave the EVM contract as-is and simply deploy a simple BridgeRouter defining contract to the EVM host account
2. Normally, we cannot re-initialize a contract but we would need a way to initialize the BridgeRouter resource in the EVM account.
   a. If we proceed with the alternative in 1.a, this wouldn't be a problem as the resource could be initialized on the new contract's deployment
   b. If we move forward with the EVM contract as proposed in this PR, is there a way we could execute an initialization block to remove the need for the public initBridgeRouter method and just include setup in the contract's init block?

You can see changes in context in onflow/flow-evm-bridge#61

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.47%. Comparing base (cb89118) to head (2c7c1c2).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5821      +/-   ##
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+ Coverage   55.69%   60.47%   +4.77%     
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  Files        1094      924     -170     
  Lines       85646    68547   -17099     
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- Hits        47704    41452    -6252     
+ Misses      33328    23393    -9935     
+ Partials     4614     3702     -912     

Flag	Coverage Δ
unittests	60.47% <ø> (+4.77%)	⬆️

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