Accept a config blob alongside the "changes" slice when committing

When committing containers to create new images, accept a container
config blob being passed in the body of the API request by adding a
Config field to our API structures.  Populate it from the body of
requests that we receive, and use its contents as the body of requests
that we make.

Make the libpod commit endpoint split changes values at newlines, just
like the compat endpoint does.

Pass both the config blob and the "changes" slice to buildah's Commit()
API, so that it can handle cases where they overlap or conflict.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

docs/source/markdown/podman-commit.1.md

@@ -36,6 +36,13 @@ Apply the following possible instructions to the created image:
 
 Can be set multiple times.
 
+#### **--config**=*ConfigBlobFile*
+
+Merge the container configuration from the specified file into the configuration for the image
+as it is being committed. The file contents should be a JSON-encoded version of
+a Schema2Config structure, which is defined at
+https://github.com/containers/image/blob/v5.29.0/manifest/docker_schema2.go#L67.
+
 #### **--format**, **-f**=**oci** | *docker*
 
 Set the format of the image manifest and metadata.  The currently supported formats are **oci** and *docker*.\