feat: add GlideAggregate (Stats API) support

[orwellsanimal]

Summary

Implements GlideAggregate, a REST wrapper over /api/now/stats/{table} that mirrors the server-side GlideAggregate. This picks up the design discussed and approved in #145.

ga = client.GlideAggregate('incident')
ga.add_active_query()
ga.add_aggregate('COUNT')
ga.add_aggregate('AVG', 'reassignment_count')
ga.group_by('priority')
ga.query()
for row in ga:
    print(row.get_value('priority'), row.get_aggregate('COUNT'))

Design (per #145)

• class GlideAggregate(Query) — inherits the query builder (add_query, add_active_query, add_null_query, add_not_null_query) rather than GlideRecord, matching the JoinQuery / RLQuery precedent and avoiding the CRUD surface that doesn't apply to aggregates. Result-cursor methods live on the class because the Stats wire shape (groupby_fields[] + stats{}) differs from a Table record.
• StatsAPI — added as a peer alongside TableAPI / AttachmentAPI / BatchAPI, sharing API._send (and therefore the 401/403/404 → exception mapping).
• client.GlideAggregate(table) factory, mirroring client.GlideRecord(table).
• Async sibling deferred to a follow-up PR.

Notes from building against a live instance

• STDDEV omitted — server-side GlideAggregate supports it, but the REST Stats API doesn't expose sysparm_stddev_fields (only count/sum/avg/min/max). Left out with a docstring note rather than faked client-side.
• order_by_aggregate is server-side via sysparm_order_by=AVG^field / COUNT.
• HAVING uses the caret form aggregate^field^operator^value (field required even for COUNT), multiple clauses joined by ,.
• set_display_value(True) sends sysparm_display_value=all, so both raw (get_value) and label (get_display_value) are available from a single query.

Files

• pysnc/aggregate.py — GlideAggregate + AggregateType
• pysnc/client.py — factory method + StatsAPI
• pysnc/__init__.py — export
• test/test_snc_aggregate.py — integration tests (mirror the test_snc_* style)
• docs/api.rst — document the new classes
• README.md — moved out of "Feature Wants and TODO" + added a Quick Start

Test plan

• pytest test/test_snc_aggregate.py — 11 integration tests against a live instance
• full suite green, mypy clean
• tests skipTest where they depend on optional demo data (e.g. ITAM assets) so they don't false-fail on instances without it

Refs #145