Switch filters to use rfc-3339 for time string encoding

[Cali0707]

Fixes #7465

Currently, we are seeing some inconsistency on time comparisons with filters. This is because we are using a string representation of the time which is explicitly marked as:

The returned string is meant for debugging; for a stable serialized representation, use t.MarshalText, t.MarshalBinary, or t.Format with an explicit format string.

CloudEvents spec states that we should use RFC-3339 for encoding timestamps as strings:
https://github.com/cloudevents/spec/blob/130ba0d183f5e45c1d141f5c1f272cf71d898623/cloudevents/spec.md?plain=1#L244

This PR switches our time.Time.String calls with code that converts the time.Time to RFC-3339 compliant strings.

Proposed Changes

• Use RFC 3339 encoding for all time strings in filter comparisons

Pre-review Checklist

• At least 80% unit test coverage
• E2E tests for any new behavior
• Docs PR for any user-facing impact
• Spec PR for any new API feature
• Conformance test for any change to the spec

Release Note

Filters now use RFC-3339 compliant string encodings for attributes of type time.

Docs

[Cali0707]

/cc @pierDipi

should we backport this?

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (64b4847) 76.79% compared to head (ac94eea) 76.68%.
Report is 18 commits behind head on main.

Files	Patch %	Lines
pkg/eventfilter/attributes/filter.go	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7466      +/-   ##
==========================================
- Coverage   76.79%   76.68%   -0.11%     
==========================================
  Files         253      253              
  Lines       14098    14173      +75     
==========================================
+ Hits        10826    10869      +43     
- Misses       2732     2751      +19     
- Partials      540      553      +13     

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[Cali0707]

/hold until I add some tests to make sure the issue is resolved

[pierDipi]

/cc @pierDipi

should we backport this?

Yes

[Cali0707]

/unhold
Per conversation here, we won't be backporting this fix as it will be a breaking change

[Cali0707]

/retest-required

[pierDipi]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Cali0707, pierDipi

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[Cali0707]

/retest-required

[Cali0707]

/test reconciler-tests