Add solution: 35_basic_key_value_store

[zhravan]

Goal: Implement the solution for exercise "$ex".

Acceptance criteria:

• Implement code in internal/exercises/templates/$ex
• All tests pass for $ex (go test ./...)
• Add/adjust docs or comments if needed
• Cross-link this issue in the PR description

Notes:

• Keep code idiomatic and simple.
• Include edge cases and table-driven tests where relevant.

[zhravan]

Tracking epic: #9