feat: allow appeal creation to continue on creator details fetch error

[rahmatrhd]

• allow_creator_details_failure is a flag that lets the appeal creation to continue when the request to the identity provider (Policy.IAM) fails. If this is set to true and request to the identity provider fails (4xx or 5xx), the value of creator field in the appeal will be nil. Note: any expression that tries to access $appeal.creator.* is still evaluated as usual, it might need to have proper nil checking to avoid accessing nil value.

closes #30

proton PR: goto/proton#21

[coveralls]

Pull Request Test Coverage Report for Build 5030951402

• 4 of 5 (80.0%) changed or added relevant lines in 2 files are covered.
• 240 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.003%) to 75.933%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/appeal/service.go	2	3	66.67%

Files with Coverage Reduction	New Missed Lines	%
api/handler/v1beta1/appeal.go	5	95.45%
domain/policy.go	13	76.47%
core/appeal/service.go	110	81.59%
plugins/providers/metabase/client.go	112	68.5%

Totals
Change from base Build 4922259145:	-0.003%
Covered Lines:	8118
Relevant Lines:	10691

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💛 - Coveralls