AGENT-848: add node-joiner cli tool main

[andfasano]

This patch introduces the node-joiner tool main with (empty) commands entry points. Since the commands will reuse the existing assets, a new AssetsFetcher interface and struct has been introduced to allow sharing the same asset fetcher code already used by the create and agent commands in the default installer.

[openshift-ci-robot]

@andfasano: This pull request references AGENT-848 which is a valid jira issue.

In response to this:

This patch introduces the node-joiner tool main with (empty) commands entry points. Since the commands will reuse the existing assets, a new AssetsFetcher interface and struct has been introduced to allow sharing the same asset fetcher code already used by the create and agent commands in the default installer.

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[andfasano]

cc @rwsu

[rwsu]

Besides the lint and fmt issues, this lgtm.

[patrickdillon]

would it make sense to have storeImpl implement AssetFetcher? It's an existing struct that contains this data structure.

installer/pkg/asset/store/store.go

Lines 47 to 51 in ba9211c

	type storeImpl struct {
	directory string
	assets map[reflect.Type]*assetState
	stateFileAssets map[string]json.RawMessage
	fileFetcher asset.FileFetcher

[andfasano]

I also initially had a look at it, but given the amount of stuff/responsabilities already implemented by storeImpl my preference was for a more isolated/separated approach, to avoid overload it too much.

[openshift-ci]

@andfasano: all tests passed!

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[rwsu]

/lgtm

[sadasu]

/approve

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sadasu

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