Clean up getting substitution expressions

[jerop]

Changes

Prior to this change, the functions for fetching substitution expressions were inconsistent in the codebase.

This change updates the functions for fetching substitition expressions from Parameters and Pipeline Results such that they are member functions, in line with related functions.

/kind cleanup

Submitter Checklist

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

- [ ] Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps

• Has Tests included if any functionality added or changed
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• 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 contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[jerop]

/test check-pr-has-kind-label

[tekton-robot]

@jerop: The specified target(s) for /test were not found.
The following commands are available to trigger required jobs:

• /test pull-tekton-pipeline-alpha-integration-tests
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[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1/param_types.go	98.7%	98.4%	-0.4
pkg/apis/pipeline/v1/pipeline_types.go	84.8%	87.2%	2.3
pkg/apis/pipeline/v1/resultref.go	98.2%	100.0%	1.8

[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1/param_types.go	98.7%	98.4%	-0.4
pkg/apis/pipeline/v1/pipeline_types.go	84.8%	87.2%	2.3
pkg/apis/pipeline/v1/resultref.go	98.2%	100.0%	1.8

[jerop]

/retest

[chitrangpatel]

/assign

[Yongxuanzhang]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lbernick

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