Safe-init

New features
- Safe Init now allows you to work around the crazy 4KB limit on environment variables in AWS Lambda. Just put some of your environment variables in a JSON file (`.env.json` by default) and Safe Init will take care of the rest.

Improvements
- Safe Init will now fall back to fetching secrets from AWS Secrets Manager if fetching from Redis throws any exception.

Improvements
- AWS Secrets resolution will use batch method to reduce number of API calls and network round trips.

Improvements
- Safe Init will no longer add the configured AWS Secrets Manager secret ARN prefix to the specified value if it already contains a prefix.

BREAKING CHANGES
- Setting Slack webhook URLs and custom loggers using monkey patching wasn't working reliably and is no longer supported. Instead, use ContextVars to set your desired values and Safe Init will read them automatically.

Improvements
- Fixed Slack notifications not being sent when Safe Init couldn't gather enough information to identify function execution context.

Improvements
- Added a new option (`SAFE_INIT_ALWAYS_NOTIFY_SLACK`) that enables Safe Init to notify Slack about failures even if a Sentry notification has been sent successfully.