Add sso-support

[antonosmond]

Currently the SDK doesn't support SSO tokens for authentication.
This is an attempt to add support for SSO tokens to address this issue by fetching the token from the CLI SSO cache and exchanging it for a set of temporary credentials via the sso.GetRoleCredentials method.
I'm not really sure if this is a suitable approach (I tried to copy roughly what the WebIdentityProvider does) but this does seem to work as I expect however I'd welcome help and feedback to get this PR moving forwards.
There's still a fair bit to do e.g. adding tests but I wanted to get some feedback before investing too much time in this.
Thanks!

[borgoat]

Hey @antonosmond !
I just wanted to say that I tested the code and it works fine for me.
The only part that I find counter-intuitive here (but I guess this is more a question to the maintainers) is that SSO credentials only get loaded if AWS_SDK_LOAD_CONFIG is set. I believe this differs from the default behaviour of Boto?

From the Go SDK docs:

By default NewSession will only load credentials from the shared credentials file (~/.aws/credentials). If the AWS_SDK_LOAD_CONFIG environment variable is set to a truthy value the Session will be created from the configuration values from the shared config (~/.aws/config) and shared credentials (~/.aws/credentials) files. Using the NewSessionWithOptions with SharedConfigState set to SharedConfigEnable will create the session as if the AWS_SDK_LOAD_CONFIG environment variable was set.

[mforutan]

This is almost all we need to switch to AWS SSO. what is the next step?

[skmcgrail]

Sorry for the delay in this feature becoming available within this SDK. This feature work was completed in #3755 and was released a v1.37.0.