Add Nextflow schema and parameter extraction workaround technique

[gwright99]

The tw cli does not yet offer a native way to extract the nextflow schema file and associated parameters from Tower. A workaround is possible via direct call to the Tower API. Document this is an 'Workaround' section of the CLI Usage page.

Command: curl '${TOWER_SERVER_URL}/api/pipelines/${PIPELINE_ID}/schema?workspaceId=${TOWER_WORKSPACE_ID}&attributes=schema,params' -H "Accept: application/json" -H "Authorization: Bearer ${TOWER_ACCESS_TOKEN}"

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