add gnmi get + helpers

Description

• Short description here, including what changes have been made. If relevant, copy todo list from the ticket here *

Closes Add ticket reference here

Reviewers list

• @marcin.chron

Self Check:

Strike through any lines that are not applicable (~~line~~) then check the box

• Correct, in line with design
• Sufficient test cases (reproduces the bug/tests the requested feature)
• Code is clear and sufficiently documented (classes, methods and function docs, document important assumptions, link to reference materials)
• Type annotations
• Issue is in the review column
• MR is assigned to first reviewer