Update feeHistory exchangeRate logic to use MirrorNode

[Nana-EC]

Problem

Currently the feeHistory call uses the mocked mirrorNode.getFeeHistory()
This includes mocked logic that shouldn't be relied on to get the most recent block

Solution

Update this to use a valid implementation
Given our fees don't change often result should be as follows

{
  baseFeePerGas: Array of all the fees.servicedata.gas for each record in weibars
  gasUsedRatio: Array of n - m  values of 0.5 i.e. [0.5, 0.5, 0.5 ...]
  oldestBlock: n
}

Current the mirrorNode file has similar logic. Adopt this and move into eth.ts

Alternatives

No response