Improve the error message when URL form encoding is received.

[bigkevmcd]

Changes

Improve the error message when URL form encoding is received.

It's a common mistake to use the default form-encoding hook format when setting
up a GitHub hook, this detects this case with an explicit error message.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
• Commit messages follow commit message best practices
• Release notes block has been filled in or deleted (only if no user facing changes)

See the contribution guide for more details.

Release Notes

The GitHub interceptor now detects form-encoded hook requests and returns an explicit error for this case rather than the generic JSON parsing error.

[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/github/github.go	85.2%	86.2%	1.0

[afrittoli]

Nice! Thank you!
/lgtm

[dibyom]

Nice!
/kind feature
/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dibyom

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