feat(cli): bulk export

[Duologic]

This ports the functionality for bulk exporting Tanka environments back from our internal CI/CD tooling.

The gist:

• moved generalized logic from cmd/tk to pkg/tanka
• add ParseParallel to evaluate envs in parallel
• tk env list uses ParseParallel
• tk export --format now takes {{env.<...>}} and fills it with the tanka.dev/Environment object
• tk export can export multiple environments and find them recursively with --recursive
• tk export creates a manifest.json file to map files back to an environment
• tk export can use -l <labelFilters>, similar to tk env list and kubectl

Examples:

# Format based on environment {{env.<...>}}
$ tk export exportDir environments/dev/ --format '{{env.metadata.labels.cluster}}/{{env.spec.namespace}}//{{.kind}}-{{.metadata.name}}'
# Export multiple environments
$ tk export exportDir environments/dev/ environments/qa/
# Recursive export
$ tk export exportDir environments/ --recursive
# Recursive export with labelSelector
$ tk export exportDir environments/ -r -l team=infra

Note: this changes tk export, <exportDir> now comes before the <environment>.

[malcolmholmes]

I think the syntax should be to expect a dir to be a single environment, or if -r is specified, recursively look for environments. This gives a clearer expectation of what the command will do - means people must explicitly request the recursive behaviour.

[sh0rez]

Took an early look, like the direction this takes.

Left some notes, will do a in-depth review later

[jvrplmlmn]

Some nits from an initial review, will go later for a second more in-depth round. Looking good 👌

[jvrplmlmn]

There is some duplicated bits that exist on the cmd/tk/util.go:writeJSON function, if we move that under pkg/... we could reuse it here. Something like:

                path := filepath.Join(to, manifestFile)
		if err := os.MkdirAll(filepath.Dir(path), 0700); err != nil {
			return fmt.Errorf("creating filepath '%s': %s", filepath.Dir(path), err)
		}
                 if err := <import>.WriteJSON(fileToEnv, path); err != nil {
			return err
		}

[Duologic]

I did a quick grep on WriteFile and found multiple very similar use cases. I propose consolidating that in a separate PR.

[Duologic]

I consolidated this bit in export.go, we can consolidate further in another PR.

[Duologic]

Rebased.

[sh0rez]

This change possibly breaks existing scripts using tk export. Is it possible to keep the old functionality intact by default?