implementing string interning to optimize resource usage for storing …

…and processing non-indexed labels (#10044)

**What this PR does / why we need it**:
In PR #9700, we added support for storing non-indexed labels in chunks.
This PR optimizes resource usage for storing and processing non-indexed
labels by doing string interning. We will store deduped label names and
values as a list in chunks
in the newly added non-indexed labels section. The labels would then be
referenced in blocks by their index(called symbols).

Additionally, I have started the convention of writing lengths of
sections with their offsets within chunks, making it easier to introduce
new sections. The section offsets and lengths would be stored at the end
of the chunk, similar to
[TOC](https://ganeshvernekar.com/blog/prometheus-tsdb-persistent-block-and-its-index/#a-toc)
in TSDB.

**Checklist**
- [x] Tests updated