[Add] Get Accounts and Balances for GasToken

[cschuchardt88]

Description

--For Offline Usage--
Allows for SDK users to get the accounts of Gas holders with their balances.

Change Log

• Added extension methods GetAccounts and BalanceOf on class GasToken.

Type of change

• Optimization (the change is only an optimization)
• Style (the change is only a code style for better maintenance or standard purpose)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• TestGetAccounts
• TestBalanceOf

Test Configuration:

• Unit Tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[AnnaShaleva]

#3610 (comment) is relevant for this PR as well, but if you don't think so, then it also LGTM.