[4.x] Fix YAML date coercion

[GrahamCampbell]

This supersedes #439 and keeps its commit from @mklenbw as it was, rebased onto 4.x, followed by one commit with the changes that came out of review.

Unquoted date-shaped scalars like an IAM policy Version were parsed into Date objects and serialized into compiled templates as ISO 8601 strings, a date behind a file variable failed the whole configuration resolution, and date-shaped mapping keys became timezone-dependent strings. Quoted and !!str values were never affected. The schema change now also re-registers the timestamp type as explicit, so a deliberate !!timestamp tag keeps constructing a Date for consumers that read files through the utils and parser APIs instead of hard-failing with an unknown tag.

The same schema is applied on the two parse paths the original commit did not reach: serverless.utils.readFileSync previously used the date-coercing default schema for any file without CloudFormation shorthand tags, and yamlParser.parse loaded external $ref documents with no schema at all, which also made shorthand tags silently fail to resolve there. One deliberate tightening: an unquoted date-only s3 lifecycle ExpirationDate or TransitionDate previously slipped through validation via the coercion and now fails it explicitly; the fix is to write the full timestamp. A companion pull request applies the same change to 3.x.