feat: support custom placeholder delimiters via workload annotations

[Abhishek9639]

Description

Adds opt-in support for custom placeholder delimiters in file content expansion, via two workload-level annotations under the existing compose.score.dev/experiment-* namespace. This is the experimental implementation of score-spec/spec#108. score-go is intentionally untouched, so the experiment lives entirely inside internal/compose and is cheap to walk away from if it doesn't pan out. That direction was agreed at the community meeting with @chris-stephenson.

What does this PR do?

Two new annotations:

• compose.score.dev/experiment-placeholder-start
• compose.score.dev/experiment-placeholder-end

When both are set on a workload, file content is expanded using the given delimiter pair instead of the default ${...} syntax. Both are required together. Setting only one of them, or leaving either value empty, produces a clear error up front. noExpand: true on a file still takes priority, and binaryContent is never expanded.

The substitution is done by a small unexported helper, substituteWithDelimiters, that builds a regex from regexp.QuoteMeta(start) + "(.*?)" + regexp.QuoteMeta(end) and runs ReplaceAllStringFunc. No escape mechanism in this version. Reasoning is on the parent issue: a doubled-prefix escape doesn't extend cleanly to asymmetric pairs like << / >> or <%{ / }%>, and a backslash escape was set aside until someone hits the problem in practice.

Example use case (Spring Boot properties, where literal ${...} needs to survive so Spring can resolve it at runtime):

apiVersion: score.dev/v1b1
metadata:
  name: spring-app
  annotations:
    compose.score.dev/experiment-placeholder-start: "<%{"
    compose.score.dev/experiment-placeholder-end: "}%>"
containers:
  app:
    image: my-spring-app
    files:
      - target: /app/application.properties
        content: |
          spring.datasource.url=jdbc:postgresql://${DB_HOST}:${DB_PORT}/${DB_NAME}
          app.name=<%{metadata.name}%>

Tests added for: annotation set with %{ }, asymmetric delimiters <%{ }%>, unset/default behaviour, noExpand priority, only-start error, only-end error, empty-value error. examples/03-files/README.md has a new section documenting the annotations.

Refs: score-spec/spec#108

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've signed off with an email address that matches the commit author.

[Abhishek9639]

Hey @chris-stephenson, @mathieu-benoit, opening this for review.

This is the score-compose-only implementation of score-spec/spec#108, following what we agreed at the meeting. score-go is intentionally untouched: the substitution is done by a small unexported helper inside internal/compose so the whole experiment lives in one place.

The behaviour matches what was in the closed score-go PR (score-spec/score-go#188) two annotations (-start / -end), both required together, validation on missing/empty values, noExpand: true still wins, binaryContent untouched. Tests cover the same cases.

If this works out in practice, we can revisit a proper score-go API later.

Could one of you approve the workflows so CI can run? First PR from me on this fork too.
Thanks!