Add request_source field

[Gutts-n]

What changed

Records a new request_source field on every analytics event emitted by
AnalyticsMiddleware, read from the Request-Source request header.
None when the header is absent (the common case).

Why

Part of the NESO API-usage-metrics work (neso-nextgen-shared#152). Data
Explorer already sends Request-Source: data-explorer on its calls to this
service (see neso-nextgen-shared#145). Without capturing that header in the
event payload, there is no way to tell Data Explorer traffic apart from
genuine external API use downstream, so it inflates the usage figures.

This only adds the field to the event shape - filtering it out of the
dashboards happens in neso-monitoring. Same change was also made to
ckanext-analytics for the CKAN Action API side (datopian/ckanext-analytics#1),
since the two services feed one shared BigQuery table.

Verified

• pytest - 481 passed.
• ruff check - clean.
• Added two new tests: header present -> value recorded, header absent ->
  None.

Not verified: end-to-end against a live pipeline (Loki -> BigQuery -> Grafana).

[coderabbitai]

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