fix: add retries when writing to redis and resp headers const

[vigith]

• Redis client internally has auto-reconnects but we have to retry in case of failures.
• Move headers as consts

Till we have CI integrated

serving@vigith$ cargo test --all-features
   Compiling serve v0.1.0 (/Users/vigith/code/git/numaproj/numaflow/serving)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 12.38s
     Running unittests src/main.rs (target/debug/deps/serve-285f01f1434f002b)

running 33 tests
test app::callback::store::memstore::tests::test_save_invalid_callback ... ok
test app::callback::store::memstore::tests::test_retrieve_datum_no_entry ... ok
test app::callback::store::memstore::tests::test_save_invalid_datum ... ok
test app::callback::store::memstore::tests::test_retrieve_callbacks_no_entry ... ok
test app::callback::store::memstore::tests::test_save_and_retrieve_datum ... ok
test app::callback::store::memstore::tests::test_save_and_retrieve_callbacks ... ok
test app::callback::state::tests::test_retrieve_subgraph_from_storage_no_entry ... ok
test app::callback::state::tests::test_retrieve_saved_no_entry ... ok
test app::callback::state::tests::test_insert_callback_requests_invalid_id ... ok
test app::tests::test_health_check ... ok
test app::tracker::tests::test_generate_subgraph ... ok
test app::callback::state::tests::test_state ... ok
test app::tracker::tests::test_generate_subgraph_from_callbacks_with_invalid_vertex ... ok
test app::tracker::tests::test_no_subgraph ... ok
test app::message_path::tests::test_message_path_not_present ... ok
test app::callback::tests::test_callback_failure ... ok
test app::callback::tests::test_callback_success ... ok
test metrics::tests::test_capture_metrics ... ok
test config::tests::test_config ... ok
test app::callback::tests::test_without_id_header ... ok
test app::callback::tests::test_callback_save ... ok
test app::tracker::tests::test_complex_dropped_message ... ok
test app::tracker::tests::test_generate_subgraph_complex ... ok
test app::tracker::tests::test_simple_cycle_pipeline ... ok
test app::tracker::tests::test_simple_dropped_message ... ok
test app::tests::test_start_main_server ... ok
test app::callback::store::redisstore::tests::test_redis_store ... ok
test app::direct_proxy::tests::test_direct_proxy ... ok
test app::jetstream_proxy::tests::test_sync_publish_serve ... ok
test app::jetstream_proxy::tests::test_sync_publish ... ok
test app::jetstream_proxy::tests::test_async_publish ... ok
test app::tests::test_setup_app ... ok
test metrics::tests::test_start_metrics_server ... ok

test result: ok. 33 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.21s

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 56.80%. Comparing base (1fc41e9) to head (408920a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1766 +/- ## ========================================== - Coverage 56.98% 56.80% -0.18% ========================================== Files 218 218 Lines 17581 17581 ========================================== - Hits 10018 9987 -31 - Misses 6710 6738 +28 - Partials 853 856 +3 ```

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