Add warning for Source.spec.ceOverride.extension names that are longer than 20 characters and with uppercase letters

[gabo1208]

Problem
As stated on the Cloud Events Spec
the attribute naming convention suggest (SHOULD, not MUST) that the names are no longer than 20 characters. We should add at maybe a warning for that cases, even when this does not imply an error. Also, when there are keys with upper case letters there could be a collision (cause the keys get converted to lower-case), in that keys the value corresponds with the last defined key (generally)

Persona:
Producers, consumers and integrators

Exit Criteria
When you set an extension name longer than 20 characters on any source.spec.ceOverride.extensions extension, the logs show a warning about that extension name

Additional context (optional)
Here should be any warning or validation related to sources 😉

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[pierDipi]

/remove-lifecycle stale
/triage accepted
/reopen

[knative-prow-robot]

@pierDipi: Reopened this issue.

In response to this:

/remove-lifecycle stale
/triage accepted
/reopen

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