simple plugin concept

[mandelsoft]

Alpha version

Using plugins it is possible now to provide additional access methods by separate executables found in a ocm plugin directory.

The plugin exectutable must implement a set of commands:

• info The info command must return a JSON object describing the the plugin and its capabilities. So far, it might only provide access methods. It is intended to extend this by downloader and uploader implementations.
• accessmethod all access relevant commands are available as sub commands for the accessmethod command
  • validate <spec> a given access specification is validated. It returns additional derived metadata for the given specification
    • a human readable info text
    • the consumer id required to determine optional credentials.
    • the media type, either derived from the spec or a constant.
    • a hint usable as reference name for local blob to reconstruct a useful name for exploded imports.
  • get -c <creds> <spec> dump the provided content to stdout
• upload (not yet used) all upload relevant commands are available as sub commands for the upload command
  • validate <name> <repo spec> a given target specification is validated. It returns additional derived metadata for the given specification
  • put -c <creds> -a <artifacttype> -m <mediatype> -h <hint> <name> <repo spec> intended for the uploader support It reads the data from stdin and puts a resulting access spec as JSON on stdout

The library now provides a command line frame for a plugin implementation, which must implement a plugin and accessmethod interface.

The ocm CLI now provides information about configured plugins.

The OCM context can be configured to register the capabilities of detected plugins, which make the additional extensions available for the regular API usage.

The help command also reflects extensions provided by plugins, now.

What this PR does / why we need it:

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

[yitsushi]

Can we refactor the whole build target to be less chaotic, for example:

NOW         := $(shell date --rfc-3339=seconds | sed 's/ /T/')
BUILD_FLAGS := "-s -w \
	-X github.com/open-component-model/ocm/pkg/version.gitVersion=$(EFFECTIVE_VERSION) \
	-X github.com/open-component-model/ocm/pkg/version.gitTreeState=$(GIT_TREE_STATE) \
	-X github.com/open-component-model/ocm/pkg/version.gitCommit=$(COMMIT) \
	-X github.com/open-component-model/ocm/pkg/version.buildDate=$(NOW)"

build: ${SOURCES}
	mkdir -p bin
	go build -ldflags $(BUILD_FLAGS) -o bin/ocm ./cmds/ocm
	go build -ldflags $(BUILD_FLAGS) -o bin/helminstaller ./cmds/helminstaller
	go build -ldflags $(BUILD_FLAGS) -o bin/demo ./cmds/demoplugin

[mandelsoft]

sure, good idea.

[mandelsoft]

done

[mandelsoft]

Unfortunately the pflag string-to-string is not compatible with my StringMap option variant. I had to fix something after the rebase.