add step_id index to artifacts table

[ykeremy]

:rocket:	This description was created by Ellipsis for commit 594ae558aadcd92d1ab1845577edc05c6ef0ee4d

Summary:

Added index to step_id column in artifacts table for improved query performance, including updates to alembic version files and ArtifactModel.

Key points:

• Added index to step_id column in artifacts table for improved query performance.
• Added index to step_id column in artifacts table in alembic/versions/2024_07_17_2216-199ea18858a3_add_step_id_index_to_artifacts_table.py.
• Modified alembic/versions/2024_07_17_2227-94bc3829eed6_add_step_id_index_to_artifacts_table.py to create and drop the index.
• Updated ArtifactModel in skyvern/forge/sdk/db/models.py to include index on step_id column.

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