LocalStack support

[SizZiKe]

Issue #, if available:
#48
Description of changes:
Adding the ability to override the sts, secretsmanager, and ssm endpoints separately (localstack feature), or to set a baseEndpoint if all services are hosted on the same URL.

I won't pretend to be a go expert, but the tests for server are already broken. I'd be happy if someone can guide me to write tests for this, but it's working ;)

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[dlsniper]

I would like to see this merged as I'm facing an issue requiring this.
@SizZiKe are you still interested in this? I can help out if required to make everything work. If not, I can fork it and send a new PR.
@simonmarty is there any chance to get this merged if everything is solved?

Thank you.

[OS-rafaelduarte]

Any updates?

[dnlopes]

I'm also interested in this feature. Having the ability to test stuff without interacting with AWS is really useful for local tests. Localstack would fit this use case perfectly.

[simonmarty]

Can you use environment variables for this? AWS_ENDPOINT_URL_SECRETSMANAGER

https://docs.aws.amazon.com/sdkref/latest/guide/feature-ss-endpoints.html

[sjones4]

You cannot use the environment variables currently as the the provider uses aws sdk for go v1 which does not support service-specific endpoints.

https://docs.aws.amazon.com/sdkref/latest/guide/feature-ss-endpoints.html#ss-endpoints-sdk-compat

In this fork/branch there is an endpoint resolver registered that uses compatible environment variables (AWS_ENDPOINT_URL_SECRETS_MANAGER , AWS_ENDPOINT_URL_STS ):

sjones4@9a3a828

This may be a good approach to use temporarily until the v2 sdk is used.