Unify Go struct field names with YAML/JSON field names

[imjasonh]

Changes

• The GitSource type is renamed to Source and moved to source.go
• The StrategyRef type is renamed to Strategy
• The StrategyRef field is renamed to Strategy
• The BuilderImage field is renamed to Builder
• The ImageURL field is renamed to Image
• SecretRef fields in various places are renamed to Credentials
• Some small godoc comment improvements

This is not intended to be a breaking API change for YAML/JSON users (incl objects stored in etcd). This will require updates to clients using generated Go clients to interact with the API, but these changes should be minimal

#516

/kind cleanup

Submitter Checklist

• [n/a] Includes tests if functionality changed/was added
• [n/a] Includes docs if changes are user-facing
• [y] Set a kind label on this PR
• [y] Release notes block has been filled in, or marked NONE

See the contributor guide
for details on coding conventions, github and prow interactions, and the code review process.

Release Notes

Update Go type and field names; no user-visible changes expected.

Existing code to interact with the API is expected to work. When updating to new versions of the generated Go client, code will need to change to reference new field names.

[qu1queee]

@imjasonh can we enhance the Release Notes to include something like:

Update Go type and field names; no user-visible changes expected. This is a breaking change that require updates to clients using generated Go clients to interact with the API

I think this will affect some internal tooling we have, build-load and shipwright cli, therefore the ask.

[imjasonh]

Updated the release notes to reflect that code will need to be updated, but existing client versions should still work in the meantime.

[qu1queee]

/lgtm

[qu1queee]

/approve

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qu1queee

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [qu1queee]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment