Update API Proxy build

[damonbarry]

1. Build API Proxy on arch-specific hosts rather than using cross. We still build inside a Docker container, so we can use an arm64v8 host to build arm32v7.
2. Build multi-arch images directly by passing multiple platforms to a single docker buildx build command, then tag the arch-specific images. This is the opposite of what we previously did (build each arch-specific image, then use them to create a multi-arch image). Note that, for API Proxy, we have to do things a little differently because it still has a separate Dockerfile for arm32v7 (which is based on Debian instead of Alpine).

Azure IoT Edge PR checklist:

This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines and Best Practices

• I have read the contribution guidelines.
• Title of the pull request is clear and informative.
• Description of the pull request includes a concise summary of the enhancement or bug fix.

Testing Guidelines

• Pull request includes test coverage for the included changes.
• Description of the pull request includes
  • concise summary of tests added/modified
  • local testing done.

Draft PRs

• Open the PR in Draft mode if it is:
  • Work in progress or not intended to be merged.
  • Encountering multiple pipeline failures and working on fixes.

Note: We use the kodiakhq bot to merge PRs once the necessary checks and approvals are in place. When it merges a PR, kodiakhq converts the PR title to the commit title, PR description to the commit description, and squashes all the commits in the PR to a single commit. The net effect is that entire PR becomes a single commit. Please follow the best practices mentioned here for the PR title and description