feat: Add DynamoDB backend

[smitrob]

Summary

Adds a new odata_query.dynamo backend that converts OData $filter expressions directly to boto3 ConditionBase objects — no eval() required.

This is an additive-only change. No existing files are modified.

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Why

DynamoDB is a widely-used AWS database with no current OData backend in this library. The Django and SQLAlchemy backends produce SQL strings, but DynamoDB uses a completely different expression model (boto3 condition objects). A dedicated visitor closes that gap cleanly.

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What's Added

odata_query/dynamo/ (new backend module)

• base.py — AstToDynamoConditionVisitor — walks the OData AST and returns live boto3 ConditionBase objects at every node
• __init__.py — exports AstToDynamoConditionVisitor and apply_odata_query

Supported OData operations:

Category	Operations
Comparison	eq, ne, lt, le, gt, ge
Logical	and, or, not
Membership	in, between
String functions	contains, startswith
Attribute checks	exists, not_exists
Null handling	eq null → not_exists OR attribute_type('NULL'); ne null → exists AND NOT attribute_type('NULL')

Unsupported functions (e.g. endswith) raise UnsupportedFunctionException immediately — no silent fallback.

tests/test_dynamodb_backend.py

20 tests covering all operators, boolean logic, null semantics, nested paths, and error cases.

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Usage

from odata_query.dynamo import apply_odata_query

condition = apply_odata_query("status eq 'active' and age gt 18")
# Returns: Attr('status').eq('active') & Attr('age').gt(18)

response = table.query(
    KeyConditionExpression=Key('pk').eq('TENANT#acme'),
    FilterExpression=condition,
)

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Backward Compatibility

• ✅ No changes to ast.py, grammar.py, visitor.py, or any existing backend
• ✅ No changes to existing tests
• ✅ boto3 is an optional dependency — doesn't affect core package installs
• ✅ Fully additive — all existing imports and behavior unchanged

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Dependencies

boto3 should be added as an optional extra in pyproject.toml:

boto3 = { version = ">=1.26", optional = true }

[tool.poetry.extras]
dynamodb = ["boto3"]

I've left pyproject.toml unchanged in this PR so maintainers can apply the version pin they prefer.