[VAULT-2789] Enable AWS Auth to read credentials from named profiles

[vinay-gopalan]

This PR adds a Profile field to the AWS Client Config. This field is passed on to HashiCorp's internal awsutil package where it is used to enable the Shared Credentials provider. This PR is in conjunction with this PR in the go-secure-stdlib repo: hashicorp/go-secure-stdlib#11

Example config:

[profile default-user]
region=us-east-1
output=json
role_arn=arn:aws:iam::501359222269:role/vault_team_dev-developer

[profile dev-role-iam]
role_arn=arn:aws:iam::501359222269:role/*
role_session_name=dev-role-iam
region=us-east-1
output=json
source_profile=default-user

Example credentials:

[default-user]
aws_access_key=...
aws_secret_access_key=...
aws_session_token=...

The addition of these changes allows the use of source_profile fields that point to a named profile with credentials for assuming roles. This aims to fix:

• Unable to login using local AWS profile with role_arn / source_profile and MFA #5767
• Vault Provider Lacks IAM Assume Role Capability terraform-provider-vault#1086 (TFVP issue related to above Vault issue)