Add admin inspect values to read foreign metric values

[wu-sheng]

Add the admin inspect values sub-command

Adds swctl admin inspect values, which reads the values of metric(s) the
target OAP does not define locally ("foreign" metrics) by supplying each
metric's {valueColumn, valueType} metadata. It POSTs to the new OAP admin API
POST /inspect/values, which runs the real MQE engine over the caller-supplied
metadata and returns the native MQE ExpressionResult (the same shape the UI
renders for catalog metrics).

Usage:

swctl --admin-url http://<oap>:17128 admin inspect values \
  --expression meter_foo \
  --service-name my-svc \
  --foreign-metric meter_foo,value,LONG \
  --start "-30m" --end "0m" --step MINUTE

• The entity scope is inferred from which name flag is set
  (--service-name → Service, --service-instance-name → ServiceInstance,
  --endpoint-name → Endpoint).
• --foreign-metric name,valueColumn,valueType is repeatable — supply one per
  foreign metric referenced by the expression.

This is the value-reading companion to the existing admin inspect entities
(which enumerates a foreign metric's entities). It targets the OAP-side
POST /inspect/values admin API (Apache SkyWalking 11.0.0).

• If this is non-trivial feature, paste the links/URLs to the design doc. — N/A (feature implementation; see the OAP PR for the server side).
• Update the documentation to include this new feature. — CHANGES.md updated; command help text included.
• Tests(including UT, IT, E2E) are added to verify the new feature. — verified end-to-end against a live OAP serving /inspect/values; lint + unit tests pass. (The command's own e2e lands with the OAP API.)

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