aws: set SharedConfigEnable to true in aws.Config

[vangent]

Some earlier discussion here:
#1357 (comment)

TL;DR, users commonly "log in" to AWS using aws login, but the AWS Go libraries don't consult the resulting ~/.aws/config file unless you set the AWS_SDK_LOAD_CONFIG environment variable to true, or unless the aws.Config field SharedConfigEnable is set to true.

We should do that in URL openers and in the *Session that's in the default provider sets.