CNF-12712: Add badges and code-coverage targets

[leo8a]

This PR proposes the following:

• Updates README.md with some badges.
• Fixes some misspell, ineffassign and gofmt recommendations provided by the goreportcard.
• Adds code-coverage target to Makefile and a config file, these could be used (e.g., in the ci-job or in local by devs) at any moment.

/cc @donpenney

[donpenney]

/cc @jc-rh

[donpenney]

/lgtm

[donpenney]

/approve

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: donpenney

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/openshift-kni/lifecycle-agent/blob/main/OWNERS)~~ [donpenney] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[openshift-ci-robot]

@leo8a: This pull request references CNF-12712 which is a valid jira issue.

Warning: The referenced jira issue has an invalid target version for the target branch this PR targets: expected the task to target the "4.16.0" version, but no target version was set.

In response to [this](https://github.com/openshift-kni/lifecycle-agent/pull/514): >This PR proposes the following: > >- Updates README.md with some badges. >- Fixes some misspell, ineffassign and gofmt recommendations provided by the goreportcard. >- Adds code-coverage target to Makefile and a config file, these could be used (e.g., in the ci-job or in local by devs) at any moment. > >/cc @donpenney Instructions for interacting with me using PR comments are available [here](https://prow.ci.openshift.org/command-help?repo=openshift-kni%2Flifecycle-agent). If you have questions or suggestions related to my behavior, please file an issue against the [openshift-eng/jira-lifecycle-plugin](https://github.com/openshift-eng/jira-lifecycle-plugin/issues/new) repository.