[CI] Enhance Makefile with dependency check for 'openapi-generator' tool

[leecalcote]

Current Behavior

Contributors must have openapi-generator CLI installed on their machines in order to run some of the make targets.

Desired Behavior

Create a new dep-check make target that gracefully handles verification of the user's installation of openapi-generator client.

Implementation

1. See the dep-check` target in the meshery/meshery repo. Copy its approach.

Acceptance Tests

1. Targets check for the presence of this dependency.
2. The prerequisite check logic works on all operating systems.
3. Users missing the prerequisite are offered a URL where they can find installation instructions.

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[1012Charan]

Hey @leecalcote , I would like to work on this

[leecalcote]

As it turns out, this tool is no longer used and needs to be cleared out from the repo. So, instead we need to:

1. clear out this openapi-generator tool from the repo.
2. create a dep-check for the other api generator / validator utility instead.