feat: add athena support

[faaabi963]

This pull request introduces core support for Amazon Athena in MetricFlow, enabling Athena as a first-class SQL engine option. The implementation covers the core engine infrastructure, adapter integration, build configuration, and documentation updates, paving the way for Athena-based development and testing workflows. The most important changes are grouped below:

Athena Engine Support and SQL Rendering:

• Added ATHENA to the SqlEngine enum in metricflow/protocols/sql_client.py, including Athena-specific unsupported granularities (nanosecond, microsecond). [1] [2]
• Implemented AthenaSqlExpressionRenderer and AthenaSqlPlanRenderer in the new file metricflow/sql/render/athena.py, with Athena-specific SQL syntax for UUIDs, date/time handling, percentiles, and edge cases.

Adapter Registration and Integration:

• Updated dbt-metricflow/dbt_metricflow/cli/dbt_connectors/adapter_backed_client.py to register Athena as a supported adapter type, map it to the new SQL engine and renderer, and import the new renderer. [1] [2] [3] [4]

Build Configuration and Testing:

• Added Athena-specific Hatch environment and dependencies (dbt-athena) to pyproject.toml, and new Makefile targets for Athena-based testing and schema population. [1] [2]

Documentation Updates:

• Added Athena to the list of supported engines and updated usage/testing instructions in CLAUDE.md.
• Added a detailed implementation plan and progress tracker in ATHENA_IMPLEMENTATION_PLAN.md.

These changes lay the foundation for Athena support in MetricFlow, with further work planned for Athena-specific testing, snapshots, and integration validation.

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