feat(dapi): implement getIdentityBalance

[pshenmic]

Issue being fixed or feature implemented

GetIdentityBalance is missing from the js dapi client. @owl352 added an implementation for this query in the #2098. I pulled these changes and added a tests.

What was done?

• Added getIdentityBalance method in the js-dapi-client (feat(dapi): identityBalance nodejs dapi-client implementation #2098)
• Added unit / integration tests
• Fixed getEpochInfo request argument (feat(dapi): optional argument start for EpochsInfo #2099)

How Has This Been Tested?

In the local mode by @owl352

Breaking Changes

No

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone