fix: Reuse HttpClient instance instead of creating one per authorize …

[vikshab]

Ticket

Fixes
stackabletech#142 issue

Problem
Each call to OpaAuthorizer.authorize() creates a new HttpClient via HttpClient.newHttpClient(). Every HttpClient spawns a SelectorManager background thread and its own connection pool that are never shut down, causing unbounded thread growth under load.

Fix
Move HttpClient to an instance field initialized once in the constructor. HttpClient is thread-safe and designed to be reused — a single instance handles all concurrent requests via its internal connection pool, eliminating the thread leak

Note:
I created this branch from the realease/0.7.0 tag specifically. See available tags with commits here https://github.com/stackabletech/druid-opa-authorizer/tags

git checkout -b DIN-8-fixing-http-client-in-release_0.7.0 cb5d92e

Built a release with fix https://github.com/webflow/druid-opa-authorizer/releases/tag/release%2F0.7.0-webflow0

TODO's before closing PR

• I'm thinking we will just cherry pick our changes into main w/o merging it, since our branch was based of release/0.7.0

git checkout main
git cherry-pick 8a1b726                                                                                                                                             
git push origin main