refactor(datalayer): key AttributeMap by DataKey instead of string

[satyamg1620]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Plugins declare the data they exchange as DataKey values, but AttributeMap access took a raw string. Nothing tied a plugin's reads and writes to its Produces()/Consumes() declaration — a plugin could declare one key and use
another.

This types AttributeMap by DataKey:

Put(fwkplugin.DataKey, Cloneable)
Get(fwkplugin.DataKey) (Cloneable, bool)
Keys() []fwkplugin.DataKey

An undeclared raw-string key is now a compile error. scheduling.Endpoint restates these methods, so the change reaches every plugin call site.

Custom scalar metrics are named in configuration rather than in code, so attrmetrics.ScalarMetricDataKey derives their key from the configured name, keeping the extractor and its readers on one construction path.

Not covered: the key still isn't checked against the calling plugin's own declaration. That needs a per-plugin scoped accessor and is worth a separate issue.

Which issue(s) this PR fixes:

Fixes #1223