Pre-parse platform string with StringSliceVar (#551)

* Pre-parse platform string with StringSliceVar

This allows users to declare --platform multiple times and have the
values appended, i.e.:
  ko build --platform=linux/amd64 --platform=linux/arm64
is equivalent to
  ko build --platform=linux/amd64,linux/arm64

As a side effect, platformMatcher.spec and gobuildOpener.platforms are
now of type []string (instead of string) to maintain structure of
information from flag parsing.

* Adjust comments and styling for clarity.

* The flag --platform is now of type strings.

Internally cobra/pflag defines StringSliceVar as "strings" whereas
StringVar is defined as "string".

This change is updated by running hack/update-codegen.sh script.

* Add backwards compatibility for WithPlatforms function signature

Update comments to reflect implementation as well.

* Fix syntax failure on unit test

doc/ko_build.md

@@ -53,7 +53,7 @@ ko build IMPORTPATH... [flags]
   -j, --jobs int                 The maximum number of concurrent builds (default GOMAXPROCS)
   -L, --local                    Load into images to local docker daemon.
       --oci-layout-path string   Path to save the OCI image layout of the built images
-      --platform string          Which platform to use when pulling a multi-platform base. Format: all | <os>[/<arch>[/<variant>]][,platform]*
+      --platform strings         Which platform to use when pulling a multi-platform base. Format: all | <os>[/<arch>[/<variant>]][,platform]*
   -P, --preserve-import-paths    Whether to preserve the full import path after KO_DOCKER_REPO.
       --push                     Push images to KO_DOCKER_REPO (default true)
       --sbom string              The SBOM media type to use (none will disable SBOM synthesis and upload, also supports: spdx, go.version-m). (default "spdx")