feat: add AWS STS JWT federation provider via GetWebIdentityToken

[Turtdle]

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[Turtdle]

tested with aws lambda:

import boto3
from britive.britive import Britive


def lambda_handler(event, context):
    sts_client = boto3.client('sts')

    response = sts_client.get_web_identity_token(
        Audience=['britive'],
        DurationSeconds=300,
        SigningAlgorithm='ES384',
    )

    token = f'OIDC::{response["WebIdentityToken"]}'

    b = Britive(tenant='<tenant>', token=token)
    me = b.my_access.whoami()

    return {
        'statusCode': 200,
        'body': json.dumps({
            'authenticated_as': me['username'],
            'user_id': me['userId'],
        }),
    }
    
    ### returns ###
    
    {
  "userId": "<id>",
  "username": "aws-sts-jwt-test",
  "type": "UI",
  "user": {
    "status": "active",
    "type": "ServiceIdentity",
    "name": "aws-sts-jwt-test",
    "lastLogin": "2026-05-11T20:35:46Z",
    "userId": "<id>",
    "attributes": [
      {
        "attributeValue": "<arn>",
        "attributeId": "<id>"
      }
    ],
    ...
  }
}