Adds buildpack API to group.toml and metadata.toml

Now that we have buidpack API dependent launch logic, the launcher needs the api of each buildpack to exist in metadata.toml so it can do the right thing
* See #116 for an example (arg parsing for direct=false processes)

Other phases may have buidpack API dependent logic as well. Adding the `api` to group.toml will provide this the api of each buildpack everywhere it is needed.

Signed-off-by: Emily Casey <[email protected]>

platform.md

@@ -817,19 +817,20 @@ Where:
 
 ```toml
 group = [
-  { id = "<buildpack ID>", version = "<buildpack version>" }
+  { id = "<buildpack ID>", version = "<buildpack version>", api = "<buildpack API version>" }
 ]
 ```
 
 Where:
 
-- Both `id` and `version` MUST be present for each buildpack object in a group.
+- `id` and `version` and `api` MUST be present for each buildpack object in a group.
 
 #### `metadata.toml` (TOML)
 ```toml
 [[buildpacks]]
 id = "<buildpack ID>"
-version = "<buildpack Version>"
+version = "<buildpack version>"
+api = "<buildpack API version>"
 optional = false
 
 [[processes]]
@@ -845,7 +846,7 @@ paths = ["<app sub-path glob>"]
 ```
 
 Where:
-- Both `id` and `version` MUST be present for each buildpack.
+- `id`, `version` and `api` MUST be present for each buildpack.
 - `processes` contains the complete set of processes contributed by all buildpacks
 - `processes` contains the complete set of slice defined by all buildpacks
 - `bom` contains the Bill of Materials