mirrors: distinguish between source/binary mirror; simplify schema (#34523)

Allow the following formats:

```yaml
mirrors:
  name: <url>
```

```yaml
mirrors:
  name:
    url: s3://xyz
    access_pair: [x, y]
```

```yaml
mirrors:
  name:
    fetch: http://xyz
    push:
      url: s3://xyz
      access_pair: [x, y]
```

And reserve two new properties to indicate the mirror type (e.g.
mirror.spack.io is a source mirror, not a binary cache)

```yaml
mirrors:
  spack-public:
    source: true
    binary: false
    url: https://mirror.spack.io
```

etc/spack/defaults/mirrors.yaml

@@ -1,2 +1,4 @@
 mirrors:
-  spack-public: https://mirror.spack.io
+  spack-public:
+    binary: false
+    url: https://mirror.spack.io