Extension release process could be slimmed down

Today the release process is:
1. Create a PR, update the version.txt and extension.yaml (manually) to be the version you want to release.
2. Wait for that PR to merge
3. Run the Azure extension pipeline to create release blobs
4. Create _another_ PR, where you've run 'azd extension release', which updates the extension.json
5. Wait for that PR to merge

Each PR approval eats up some time, and is something manual that needs to be tracked. I think we could, at the very least, condense steps 3,4 and 5 into a _single_ pipeline, where the final phase writes out the extension.json. That saves me from having to submit a final PR and speeds up iteration time.

Start by tracing the Azure extension pipeline and the `azd extension release` command. Check how `version.txt`, `extension.yaml`, and `extension.json` are produced across the current steps. Done means the pipeline handles the release blobs and final `extension.json` update without requiring a second PR.

Written by the indexing model from the issue text.