feat(sdk): rs-sdk fetch current epoch

[lklimek]

Issue being fixed or feature implemented

We need a way to fetch current epoch

What was done?

Implemented ExtendedEpochInfo::fetch_current()

How Has This Been Tested?

Added unit test

Breaking Changes

None

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