Support OIDC connect

[prein]

I would like to use this action without AWS credentials, using OIDC for auth.

An example usage could look something like the following:

jobs:
  deploy:
    runs-on: ubuntu-latest
    permissions:
      id-token: write
      contents: read
    steps:

    - name: Configure AWS credentials
      uses: aws-actions/configure-aws-credentials@master
      with:
        role-to-assume: arn:aws:iam::${{ env.AWS_ACCOUNT_ID }}:role/${{ env.AWS_ROLE_NAME }}
        aws-region: ${{ env.AWS_REGION }}

    - name: Login to Amazon ECR
      id: login-ecr
      uses: aws-actions/amazon-ecr-login@v1

    - uses: actions/checkout@master

    - uses: jakejarvis/s3-sync-action@master
      with:
        args: --acl public-read --follow-symlinks --delete
      env:
        AWS_S3_BUCKET: ${{ secrets.AWS_S3_BUCKET }}
        SOURCE_DIR: 'public'      # optional: defaults to entire repository

[prein]

I think it might be redundant to #16