feat(tools): add aws sigv4 signing tool

[suhridsaran]

Description

This PR adds a new tool, 'AwsSigV4Tool', which generates AWS signature version 4 headers for HTTP requests.
It is fully compatible with the strands HTTP tool and does not require any external dependencies like boto3.
This allow strands user to securely call any AWS services with minimal setup.

The PR includes:

• The tool implementation in 'src/strands/tool/aws_sigv4.py
• Unit tests in tests/strands/tool/test_aws_sigv4.py
• Documentation in aws_sigv4.md with usage example and parameter description.

Related Issues

Documentation PR

Type of Change

New feature

Testing

• [ X] I ran hatch run prepare
• [X ] Unit test pass ('hatch test')
• [X ] Integration test pass ('hatch run test-integ')
• [ X] Verified no warning in consuming repo

Checklist

• [ X] I have read the CONTRIBUTING document
• [X ] I have added any necessary tests that prove my fix is effective or my feature works
• [X ] I have updated the documentation accordingly
• [X ] I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• [X ] My changes generate no new warnings
• [ X] Any dependent changes have been merged and published

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