Add IEventStoreOptions.IgnoreIndex(name)

[zidad]

Closes #4341.

What

Adds IEventStoreOptions.IgnoreIndex(string indexName), mirroring the existing DocumentMapping.IgnoreIndex(string indexName) for document tables. The ignore list is consulted by all three event-store tables (mt_events, mt_streams, mt_event_progression) so a single IgnoreIndex("foo") covers any of them.

Why

If you add an index on a Marten-managed event-store table outside Marten's own schema (e.g. a custom IFeatureSchema declaring CREATE INDEX ... ON mt_events USING GIN (headers jsonb_path_ops)), the schema diff sees an unmanaged index on a managed table and drops it on the next ApplyAllDatabaseChangesOnStartup(). The same gap was already solved for documents — this PR brings the event-store side to parity.

Usage

services.AddMarten(opts =>
{
    opts.Events.IgnoreIndex("mt_events_headers_gin_idx");
    opts.Events.IgnoreIndex("mt_events_data_gin_idx");
    opts.Storage.Add<EventQueryIndexFeature>();
});

Changes

• IEventStoreOptions: new IgnoreIndex(string) method + IReadOnlyList<string> IgnoredIndexes { get; }.
• IReadOnlyEventStoreOptions: same IgnoredIndexes property.
• EventGraph: backing list + idempotent add + null/whitespace guard.
• EventsTable, StreamsTable, EventProgressionTable: copy events.IgnoredIndexes onto the Weasel base Table.IgnoredIndexes (mirrors DocumentTable.cs:25-26).
• New tests under src/EventSourcingTests/ignoring_indexes_on_event_store_tables.cs (7 tests, all passing on net8/9/10 — no DB required).
• Doc snippet + section in docs/events/configuration.md.

Opened as draft per the discussion in #4341 — happy to adjust scope or naming.