Sentry Structured Logging for PHP

[cleptric]

https://develop.sentry.dev/sdk/telemetry/logs/

Sentry is adding support for structured logging. Let's add it to the Native SDKs!

1. Define the logs protocol and log envelope item in the SDK
2. Add the Public API (SDK options and methods) as per the docs
3. Make sure the the SDK follows the documented behavior
4. Attach default attributes to the SDK as per docs
5. Instrument popular logging libraries to send logs to Sentry. SDK maintainers can best decide what they should support, but we should definitely support the same integrations that we do in the SDK today.

Logs can be emitted with one of the six available log levels, trace, debug, info, warn, error and fatal.

Usage without the agent looks as follows:

logger()->info('Hello, PHP!');
logger()->flush();

With the agent, users can omit the manual flush call:

logger()->info('Hello, PHP!');

WIP PR #1813

[giggsey]

Could this support PSR-3?