Adds full proxy configuration

[gdavison]

Adds HttpsProxy and NoProxy parameters and support for HTTPS_PROXY, http_proxy, NO_PROXY, and no_proxy environment variables

Closes #133

[jar-b]

LGTM 🎉

% go test -count=1 ./...
?       github.com/hashicorp/aws-sdk-go-base/v2/configtesting   [no test files]
?       github.com/hashicorp/aws-sdk-go-base/v2/internal/awsconfig      [no test files]
?       github.com/hashicorp/aws-sdk-go-base/v2/internal/constants      [no test files]
?       github.com/hashicorp/aws-sdk-go-base/v2/internal/errs   [no test files]
?       github.com/hashicorp/aws-sdk-go-base/v2/internal/slices [no test files]
?       github.com/hashicorp/aws-sdk-go-base/v2/internal/test   [no test files]
?       github.com/hashicorp/aws-sdk-go-base/v2/mockdata        [no test files]
?       github.com/hashicorp/aws-sdk-go-base/v2/servicemocks    [no test files]
ok      github.com/hashicorp/aws-sdk-go-base/v2 4.720s
ok      github.com/hashicorp/aws-sdk-go-base/v2/diag    0.215s
ok      github.com/hashicorp/aws-sdk-go-base/v2/internal/config 0.338s
ok      github.com/hashicorp/aws-sdk-go-base/v2/internal/endpoints      0.257s
ok      github.com/hashicorp/aws-sdk-go-base/v2/internal/expand 0.251s
ok      github.com/hashicorp/aws-sdk-go-base/v2/logging 0.269s
ok      github.com/hashicorp/aws-sdk-go-base/v2/tfawserr        0.193s
ok      github.com/hashicorp/aws-sdk-go-base/v2/useragent       0.219s
ok      github.com/hashicorp/aws-sdk-go-base/v2/validation      0.265s