Introduce more exhaustive GetSession testing and resolve some endpoint customization bugs

[bflad]

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Reference: #2
Reference: hashicorp/terraform-provider-aws#9869
Reference: hashicorp/terraform#25056

Practitioner facing changes:

• Properly use custom STS endpoint during AssumeRole API calls triggered by Terraform AWS Provider and S3 Backend configurations
• Properly use custom EC2 metadata endpoint during API calls triggered by fallback credentials lookup

Other changes:

• Use Go 1.13 wrapped error handling
• Introduce centralized endpoints.Resolver for custom endpoint handling
• Introduce mock ECS credentials server
• Introduce STS AssumeRole valid and invalid API responses

[gdavison]

LGTM 🚀