plugins: add new plugin for linking and unlinking issues to a PR

[Amulyam24]

This PR adds a new plugin issue-management which has commands for linking and unlinking issues to a PR. Ref - https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue

• The commands can be used to link an issue to a PR in the current repository or in a different repository as well as handle multiple issues
• This is done by adding the supported keyword Fixes to the body of the PR if it doesn't already exist or by appending the issue to the existing Fixes line
• Supported formats are issue-number and org/repo-name#issue-number

A new plugin has been added to accommodate any existing issue commands or provide flexibility to support more issue commands in the future.

Fixes #359

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Amulyam24 Once this PR has been reviewed and has the lgtm label, please assign cjwagner for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

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[k8s-ci-robot]

@Amulyam24: The label(s) `/label do-not-merge/work-in-progress

cannot be applied. These labels are supported:api-review, tide/merge-method-merge, tide/merge-method-rebase, tide/merge-method-squash, team/katacoda, refactor, ci-short, ci-extended, ci-full. Is this label configured under labels -> additional_labelsorlabels -> restricted_labelsinplugin.yaml`?

In response to this:

/label do-not-merge/work-in-progress

Adding UT

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[Amulyam24]

/hold

WIP: Adding UT

[Amulyam24]

/unhold

[petr-muller]

I think this is pretty close, I appreciate the test coverage. Left some comment about the code structure.