Skip analytics for configured ignore header

[Gutts-n]

What changed

A request carrying the configured ignore header/value now skips analytics
entirely - not logged with a distinguishing field, just never recorded.
New config (module-level, read from env at import like the existing
CKANEXT_ANALYTICS_SERVICE):

• CKANEXT_ANALYTICS_IGNORE_HEADER - header name to check (e.g.
  Request-Source)
• CKANEXT_ANALYTICS_IGNORE_VALUES - comma-separated values that trigger
  the skip (e.g. data-explorer), matched case-insensitively

Both empty (the default) disables the check - every tracked request is
logged, same as before this PR.

Why

Part of the NESO API-usage-metrics work (neso-nextgen-shared#152). Data
Explorer sends Request-Source: data-explorer on its calls
(neso-nextgen-shared#145). Original version of this PR captured the header
into the event and left filtering to the BigQuery/dashboard layer -
@sagargg pointed out on the matching datastore
PR
that's the wrong layer: better to not log this traffic at all than log it
and filter downstream. Reworked accordingly, same change applied to
datopian/datastore#19 (the Datastore API's own analytics middleware).

Verified

• pytest ckanext/analytics/tests/test_event.py - 48 passed (ran locally
  with a minimal flask+pytest venv, --ckan-ini addopts overridden since
  no CKAN environment was available locally).
• Tests cover: header+value match skips (case-insensitive), no header sent
  still records, header present with a non-matching value still records,
  check disabled by default (unconfigured) never skips.

Not verified: end-to-end against a live CKAN + Loki + BigQuery pipeline.

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[Gutts-n]

Closing in favor of #2 - now that I have push access to this repo, opened the same branch directly against it instead of from the fork.