Add Statefulness to Custom Code Blocks

[yeldarby]

Description

Adds statefulness to custom Python code blocks in Workflows, allowing blocks to maintain state across executions.

• Local mode: Shared globals dict accessible by all custom blocks in the workflow
• Modal mode: Per-block namespaces persist within container lifetime, with shared globals dict across all blocks

Note: the globals dict is left undocumented for now while we decide whether it is necessary & desired.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How has this change been tested, please provide a testcase or example of how you tested the change?

• Custom blocks can access and modify a persistent globals dictionary
• State persists across multiple executions of the same block
• Modal containers maintain separate namespaces for each unique code block

Any specific deployment considerations

Requires deployment of the Modal app.

Docs

• Docs updated? What were the changes: Added info on this behavior to the custom python blocks docs.

[hansent]

LGTM