chore: Use floats for fee multipliers, add base multiplier

[janjakubnanista]

Description

The fee estimator for devnet-sdk had a bit of a usability issue with it - the base & tip multipliers were defined as big.Ints which meant the fees could only be increased by integer multiples. This is not great as there are plenty of numbers between 1 and 2 (some would even say an infinite amount of numbers but those people are not working with float64 limitations).

This PR fixes the issue and adds base multiplier besides the tip multiplier. Some libraries set this to 1.2 by default (ethers, viem) but to avoid any surprises, a value of 1.0 will be used for both of these by default.

On top of the changes in the logic, the functional options pattern has been used (thanks @scharissis for this)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.92%. Comparing base (f81a2e3) to head (77107be).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #14818      +/-   ##
===========================================
- Coverage    46.48%   41.92%   -4.56%     
===========================================
  Files         1046      874     -172     
  Lines        90907    80759   -10148     
===========================================
- Hits         42254    33862    -8392     
+ Misses       45547    43966    -1581     
+ Partials      3106     2931     -175     

Flag	Coverage Δ
cannon-go-tests-32	?
cannon-go-tests-64	?
contracts-bedrock-tests	?

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see 178 files with indirect coverage changes

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[scharissis]

Just flagging what I think is non-idiomatic Go; not a hard blocker for me if you need to merge soon. (But worth chatting about here or elsewhere!)

[janjakubnanista]

@scharissis made some changes, could you please re-check?