Allow not creating service accounts (serviceAccount.create=false)

Currently the chart forces you to allow it to create the service account, which is something we have a general rule not to allow vendored charts to do (our practice is to pre-create the namespaces, service accounts, and cluster role bindings using a common Helm chart).

The Buildkite agent Helm chart should allow disabling creation of the service account, and allowing specification of the pre-created one.

Locate the Buildkite agent Helm chart's service-account values and template, then inspect how the chart currently renders that account. The change is complete when users can disable service-account creation and specify an existing account, with rendered Kubernetes manifests confirming both paths.

Written by the indexing model from the issue text.