Backport of Fix a panic in connect-inject when the provided upstreams list is malformed into release/1.4.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #3956 to be assessed for backporting due to the inclusion of the label backport/1.4.x.

🚨

Warning automatic cherry-pick of commits failed. If the first commit failed,
you will see a blank no-op commit below. If at least one commit succeeded, you
will see the cherry-picked commits up to, not including, the commit where
the merge conflict occurred.

The person who merged in the original PR is:
@dhiaayachi
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: unable to process merge commit: "86466472ec0f44d0e75e8fea567a6fc03936a45c", automatic backport requires rebase workflow

The below text is copied from the body of the original PR.

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Changes proposed in this PR

• Fix a panic when parsing a malformed upstream, now a malformed upstream will be ignored
• Support multiple separators between upstreams (, , ).

How I've tested this PR

Added unit tests to test multiple scenarios including malformed upstream lists

How I expect reviewers to test this PR

N/A

Checklist

• Tests added
• CHANGELOG entry added

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Overview of commits

• 2b30a0d - 03c1887 - 4af5e68 - 4c471dd - 13c989f - 8646647