feat: Add experimental concurrent processor for logs

[cijothomas]

1. Add a new processor under experimental feature flag.
2. This is to facilitate discussions in OTel spec about adding such a component to the official spec itself.
3. This is NOT useful as-is with any existing exporters in this repo (like OTLP Log Exporter
4. This is an initial version. Next, I'll introduce ways to ensure that only those exporters that can support concurrent exporting will be paired with ConcurrentExporter. This is one possible way spec could evolve. Alternatively, existing processors can be modified to export concurrently if the exporter advertises that they are okay with that!. BatchLogProcessor also need to evolve to support concurrent exporting in the future.
5. Also modified Stress test to use the new experimental processor to compare the perf numbers with Concurrent vs Existing SimpleProcessor.
6. SimpleProcessor did not have a public new method and it did not implement event_enabled. This addresses that also, as it was required to run stress test comparison. Can do this in a separate PR it helps review easier.

Spec discussion:
open-telemetry/opentelemetry-specification#4231

[codecov]

Codecov Report

Attention: Patch coverage is 25.64103% with 29 lines in your changes missing coverage. Please review.

Project coverage is 79.6%. Comparing base (dac8bd5) to head (967ea29).

Files with missing lines	Patch %	Lines
...telemetry-sdk/src/logs/concurrent_log_processor.rs	0.0%	28 Missing ⚠️
opentelemetry-sdk/src/logs/simple_log_processor.rs	90.9%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2780     +/-   ##
=======================================
- Coverage   79.7%   79.6%   -0.1%     
=======================================
  Files        123     124      +1     
  Lines      23117   23155     +38     
=======================================
+ Hits       18428   18437      +9     
- Misses      4689    4718     +29     

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[lalitb]

LGTM.