Deprecate global AWS config as first argument.

[peteristhegreat]

Most function arguments are of the form

function s3_xxx_xxx(::AbstractAWSConfig, a, b, c; kw1="foo", kw2="bar")
     # function body
end

s3_xxx_xxx(a...; b...) =  s3_xxx_xxx(global_aws_config(), a...; b...)

Whereas AWS.jl now passes the AbstractAWSConfig as a keyword argument.

Reverse this, so that the AbstractAWSConfig is not in the main version of each function, but only in the keyword version of each function.

Here is the new form:

function s3_xxx_xxx(a, b, c; aws_config::AbstractAWSConfig=global_aws_config(), kw1="foo", kw2="bar")
     # function body
end

s3_xxx_xxx(aws_config::AbstractAWSConfig, a...; b...) =  s3_xxx_xxx(a...; aws_config=aws_config, b...)

This in general is more consistent with AWS.jl and should remain backward compatible with existing uses of AWSS3

Here is an example in AWS.jl.
https://github.com/JuliaCloud/AWS.jl/blob/master/src/services/sts.jl#L711

function get_caller_identity(; aws_config::AbstractAWSConfig=global_aws_config())
    return sts("GetCallerIdentity"; aws_config=aws_config, feature_set=SERVICE_FEATURE_SET)
end
function get_caller_identity(
    params::AbstractDict{String}; aws_config::AbstractAWSConfig=global_aws_config()
)
    return sts(
        "GetCallerIdentity", params; aws_config=aws_config, feature_set=SERVICE_FEATURE_SET
    )
end