Replace boto dependency with custom AWS4 request signer

[nspadaccino]

This PR removes the boto dependency and replaces it with a custom, lightweight AWS Signature Version 4 request signer implementation. This reduces the dependency footprint and
eliminates the need for the heavy boto library that was only used for request signing.

Changes

File	Changes
setup.py	Removed boto>=2.48.0 from install_requires
src/vinyldns/aws4auth_request_signer.py	New custom AWS4 request signer implementation
src/vinyldns/client.py	Updated to use Aws4AuthRequestSigner instead of BotoRequestSigner

Motivation

• Reduced dependencies: Removes the heavy boto package which was only used for AWS Signature V4 signing
• Lighter footprint: The custom signer uses only standard library modules (hashlib, hmac, datetime) plus requests which is already a dependency
• Better maintainability: Self-contained signing logic that's easier to understand and maintain

Implementation Details

The new Aws4AuthRequestSigner class implements AWS Signature Version 4 authentication:

• Generates canonical requests with proper URI, headers, and query string encoding
• Creates HMAC-SHA256 signing keys using the standard AWS4 key derivation
• Builds properly formatted Authorization headers
• Handles date parsing from X-Amz-Date or Date headers with fallback to current time

Testing

• Run existing test suite to verify signing works correctly
• Test against VinylDNS API endpoints