Add first and last functions.

[bigkevmcd]

Changes

This adds two new functions to the CEL interceptor functions, first and last which simplifies access JSON array values in CEL expressions.

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• [x] Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• [x] Has Tests included if any functionality added or changed
• [x] Follows the commit message standard
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• [x] Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
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Release Notes

Two new functions for the CEL interceptor, to allow easy access to the first and last elements in an array.

[tekton-robot]

The following is the coverage report on the affected files. Say /test pull-tekton-triggers-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/interceptors/cel/triggers.go	88.5%	89.7%	1.2

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dibyom

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[khrm]

/kind feature