[source-marketo] Custom fields within the leads stream

[Lucas Leadbetter (lleadbet)]

The Marketo connector currently does not support custom fields in the leads stream, as documented here: https://docs.airbyte.com/integrations/sources/marketo#supported-streams

Request:

Add support for dynamic schema generation to include custom fields in the leads stream, similar to other source connectors.

Context:

Many organizations use custom fields in Marketo to track business-specific lead attributes (e.g., industry vertical, product interest, custom scoring fields).

Without this support, users cannot sync this data.

Expected behavior:

• Connector should discover available custom fields via the Marketo API
• Custom fields should appear in the leads stream schema alongside standard fields
• Schema should refresh when new custom fields are added in Marketo