fix: fixed propagation of exceptions from upstream endpoints

[adubovik]

• fix: typo in README
• fix: debug logs for response/request weren't logged (it happened because _debug constant was evaluated before the log level was set)
• fix: merging of chat completion chunks in non-streaming wasn't working properly, e.g. object fields were concatenated which resulted in "object": "chat.completionchat.completionchat.completion". Added a test witnessing the bugfix.
• fix[examples]: en_core_web_sm model is (a) downloaded as part of the package build and (b) pre-loaded on the server start-up instead of spending time of the first request into PII interceptor
• fix[examples]: pinned versions of en_core_web_sm model (3.7.1) and spacy (3.7.5) to avoid model downloading issue on the server start up
• deps: migrated FileStorage from aiohttp to httpx, thus aiohttp is removed from the package dependencies
• deps: aiostream package dependency moved to the examples extras
• deps: migrated from aidial-sdk 0.13.0 to 0.16.0
• tests: built infrastructure for testing chains of interceptors
• tests: introduced X-UPSTREAMS header and refactored DIAL Client to ease interceptor testing
• tests: added tests that validate response headers propagation from the upstream in streaming and non-streaming scenarios
• feat: DIALException are materialised instead of being treated as normal chat completion chunks (that's what handle_streaming_errors method used to do). This led to ChatCompletionInterceptor.call_upstreams changing type to accommodate the new kind of stream values which hold DIAL Exception.
• chore: migrated from the custom _UnstructuredChunk to ArbitraryChunk from DIAL SDK