auth: support Web Identity Token File

## Problem
We can't log in AWS when using web_identity_token_file in AWS config Profile like this:

```
[default]
region=
web_identity_token_file=
role_arn=arn:aws:iam:123456789:role/role-name
```
I get the following error:
`login: failed to connect with "profile:default": Role Arn 'arn:aws:iam:123456789:role/role-name' needs to be assumed with web identity, but no role assumption callback was provided.`

The vscode and extensions run into an kubernetes cluster with an OIDC token into the web identity file

## Expected behavior
I would be logged via Web Identity Token File like the aws cli :(

No file or test is named. Start by locating the AWS profile loading and role-assumption entry points, then reproduce the configuration with a web identity token file; done means login succeeds for that profile without the reported callback error.

Written by the indexing model from the issue text.