HITL Approve UI does not correctly display object DAG parameters. These were being displayed as [object Object] (see below).

Main change (hitl.ts):

• Added support for preloading parameter values from URL query strings
• Added JSON parsing for string values that look like JSON objects
• Fixed type detection to correctly identify object parameters (instead of defaulting to string)
  Test file (hitl.test.ts):
• New test verifying object parameters are typed as 'object'

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