feat: allow policy iam config to retrieve value using expression

[rahmatrhd]

Changes:

• support expression evaluation in policy iam schema config example:

  // response from identity service
  {"managers": [{"email": "user@example.com"}]}

  # policy
  iam:
   ...
   schema:
     managers: $response.managers[0].email

  // appeal result
  Appeal{
   ...
   Creator: map[string]interface{}{
     "managers": "user@example.com"
   },
  }

[coveralls]

Pull Request Test Coverage Report for Build 4140857787

• 27 of 36 (75.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 75.287%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/appeal/service.go	27	36	75.0%
<!--	Total:	27	36	75.0%	-->

Files with Coverage Reduction	New Missed Lines	%
core/appeal/service.go	1	81.23%
<!--	Total:	1		-->

Totals
Change from base Build 4111850609:	0.03%
Covered Lines:	7674
Relevant Lines:	10193

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💛 - Coveralls