feat_: introduce erc20 balance fetcher

[dlipicar]

Add ERC20 balance fetching capabilities to the balance fetcher package.
Adjusted example app balance-fetcher-web to showcase new functionality.

[friofry]

Looks great!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 87.28%. Comparing base (5d8e347) to head (85ceaad).

Files with missing lines	Patch %	Lines
pkg/balance/fetcher/fetcher_standard.go	86.04%	4 Missing and 2 partials ⚠️
pkg/balance/fetcher/fetcher_balancescanner.go	92.10%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master       #5       +/-   ##
===========================================
+ Coverage   45.07%   87.28%   +42.21%     
===========================================
  Files           7        5        -2     
  Lines         264      236       -28     
===========================================
+ Hits          119      206       +87     
+ Misses        142       26      -116     
- Partials        3        4        +1     

Files with missing lines	Coverage Δ
pkg/balance/fetcher/fetcher.go	100.00% <100.00%> (ø)
pkg/balance/fetcher/fetcher_balancescanner.go	94.54% <92.10%> (-5.46%)	⬇️
pkg/balance/fetcher/fetcher_standard.go	90.90% <86.04%> (-9.10%)	⬇️

... and 3 files with indirect coverage changes

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[vkjr]

We have a separate package name for tests, which leads us to having functions like FetchErc20BalancesWithBalanceScanner exported. Is this a deliberate approach to not hide the internals of fetcher logic? Do we allow those functions to be called directly?

[dlipicar]

I saw no good reason not to export this function (some dev might have their own private deployment of BalanceScanner, or want to ensure that either the more efficient way is used or an error is returned), which allowed me to write the tests in a separate package (which I always find preferable to ensure we're testing behavior and not implementation).