Allow specifying AWS IAM role to assume with IAM user keys

[hhamalai]

Is your feature request related to a problem? Please describe.

Currently drone-cache support AWS users, but often the permissions are granted to roles instead of users. The usage of roles also allows universally working API access when some policies are IP restricted as described here https://aws.amazon.com/premiumsupport/knowledge-center/iam-restrict-calls-ip-addresses/

Describe the solution you'd like
Add parameter to specify role ARN to assume, use access keys to assume role with STS and retrieve credentials with role rights.