Add GitHub Action to update `aws-sdk-rust/next` branch (#1774)

# This workflow updates the `next` branch with freshly generated

# code from the latest smithy-rs and models that reside in aws-sdk-rust.

name: Update `aws-sdk-rust/next`

on:

  workflow_dispatch:

jobs:

  update-next:

    name: Update `next`

    runs-on: ubuntu-latest

    steps:

    - name: Check out `smithy-rs`

      uses: actions/checkout@v3

      with:

        repository: awslabs/smithy-rs

        ref: main

        path: smithy-rs

    - name: Check out `aws-sdk-rust`

      uses: actions/checkout@v3

      with:

        repository: awslabs/aws-sdk-rust

        ref: main

        path: aws-sdk-rust

        token: ${{ secrets.RELEASE_AUTOMATION_BOT_PAT }}

    - name: Set up JDK

      uses: actions/setup-java@v1

      with:

        java-version: 11

      # Rust is only used to `rustfmt` the generated code; doesn't need to match MSRV

    - name: Set up Rust

      uses: actions-rs/toolchain@v1

      with:

        toolchain: stable

    - name: Delete old SDK

      run: |

    - name: Generate a fresh SDK

      run: |

        WORKSPACE="$(pwd)"

        cd smithy-rs

        ./gradlew aws:sdk:assemble --info -Paws.sdk.models.path="${WORKSPACE}/aws-sdk-rust/aws-models"

    - name: Update `aws-sdk-rust/next`

      run: |

        set -eux

        cd aws-sdk-rust

        git checkout origin/main -b next

        # Delete the old SDK

        rm -rf sdk examples

        rm -f versions.toml Cargo.toml index.md

        # Copy in the new SDK

        mv ../smithy-rs/aws/sdk/build/aws-sdk/* .

        git add .

        git -c 'user.name=AWS SDK Rust Bot' -c 'user.email=aws-sdk-rust-primary@amazon.com' commit -m 'Update `aws-sdk-rust/next`' --allow-empty

        git push origin next:next --force