Add Lambda Managed Instances (LMI) / Multi-Concurrent Support

[trivenay]

_Issue #, if available: NA

_Description of changes:
This PR adds support for Lambda Managed Instances (LMI), enabling multi-concurrent execution in the Python Runtime Interface Client via multi-processing

Changes

Core Features:

• Add MultiConcurrentRunner class to handle multi-concurrent Lambda execution
• Add LambdaMultiConcurrentRuntimeClient with retry logic and error suppression for concurrent workers
• Add configuration support for AWS_LAMBDA_MAX_CONCURRENCY and socket path environment variables
• Implement stdout/stderr redirection via Unix sockets for concurrent processes

Major changes:

• awslambdaric/lambda_multi_concurrent_utils.py - Multi-concurrent runner implementation
• awslambdaric/__main__.py - Entry point routing for multi-concurrent mode
• awslambdaric/lambda_config.py - New config properties (is_multi_concurrent, lmi_socket_path)
• awslambdaric/lambda_runtime_client.py - LMI client for Runtime API

Testing

make test 

Name                                            Stmts   Miss  Cover   Missing
-----------------------------------------------------------------------------
awslambdaric/__init__.py                            1      0   100%
awslambdaric/__main__.py                           18      1    94%   33
awslambdaric/bootstrap.py                         260     11    96%   107-114, 351, 354, 357, 510-519
awslambdaric/lambda_config.py                      45      0   100%
awslambdaric/lambda_context.py                     57      4    93%   60, 80, 98, 113
awslambdaric/lambda_literals.py                     2      0   100%
awslambdaric/lambda_multi_concurrent_utils.py      31      0   100%
awslambdaric/lambda_runtime_client.py             126      5    96%   30, 116, 135, 152-153
awslambdaric/lambda_runtime_exception.py           16      0   100%
awslambdaric/lambda_runtime_hooks_runner.py        10      0   100%
awslambdaric/lambda_runtime_log_utils.py           56      0   100%
awslambdaric/lambda_runtime_marshaller.py          35      0   100%
-----------------------------------------------------------------------------
TOTAL                                             657     21    97%

Required test coverage of 90% reached. Total coverage: 96.80%

============================= 123 passed in 0.79s ==============================

_Target (OCI, Managed Runtime, both): Both

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.