Allow SPIRE-Server to work in all AWS partitions

[achaurasiaConfluent]

Current SPIRE Server aws_iid nodeattestor plugin assume that the role is in partition aws.
Due to current implementation

spire/pkg/server/plugin/nodeattestor/awsiid/client.go

Lines 80 to 82 in 0527653

	if cc.config.AssumeRole != "" {
	asssumeRoleArn = fmt.Sprintf("arn:aws:iam::%s:role/%s", accountID, cc.config.AssumeRole)
	}

spire would only work in aws partition.

Refer https://docs.aws.amazon.com/IAM/latest/UserGuide/reference-arns.html
for aws partition and arn format.

• Version: 1.7.0
• Platform: any
• Subsystem: Server node attestor plugin

[MarcosDY]

Done