chore: preparing for 1.0 RC version (#817)

1. Run release-please on 1.0-dev and set `target-branch` in the GitHub
Action.
2. Use advanced configuration to specify pre-release versioning
(`"prerelease": true` and `"prerelease-type": "alpha"`), this is going
to result in `1.0.0a0` (see
[here](https://packaging.python.org/en/latest/specifications/version-specifiers/#pre-releases)
package for `v1.0.0-alpha.0` tag.
3.
[python-publish.yml](https://github.com/a2aproject/a2a-python/blob/main/.github/workflows/python-publish.yml)
workflow was disabled as a safety measure in case release-please goes
crazy with this config.

For now do not specify `last-release-sha` explicitly
([docs](https://github.com/googleapis/release-please/blob/main/docs/manifest-releaser.md#configfile)),
will check the behavior first, changelog will have to be created
manually most likely either way.

Author: ishymko

.github/workflows/release-please.yml

@@ -2,6 +2,7 @@ on:
   push:
     branches:
       - main
+      - 1.0-dev
 
 permissions:
   contents: write
@@ -16,4 +17,6 @@ jobs:
       - uses: googleapis/release-please-action@v4
         with:
           token: ${{ secrets.A2A_BOT_PAT }}
-          release-type: python
+          target-branch: ${{ github.ref_name }}
+          config-file: release-please-config.json
+          manifest-file: .release-please-manifest.json

.release-please-manifest.json

@@ -0,0 +1 @@
+{}

release-please-config.json

@@ -0,0 +1,8 @@
+{
+  "release-type": "python",
+  "prerelease": true,
+  "prerelease-type": "alpha",
+  "packages": {
+    ".": {}
+  }
+}