[ConnectorsForge] Expanding user authentication possibilities

[marek-zuwala]

This PR expands the authentication capabilities of connectors created with ConnectorsForge. It also fixes several issues that were discovered during testing.

Advanced authentication DSL

• New AuthConfig model + object-form $authentication in the DSL supports custom headers, base64(…) encoding, and a new CUSTOM auth type for arbitrary headers.
• RestDatasourceType.
• Full backward-compat: bare-string form (e.g. "api_key") is auto-expanded.

Custom Accept header — new $accept_header DSL field, defaults to application/json.

Field types invisible in CP4D discovery UI (root cause fix)

• ForgeSchemaBuilder.toArrowField() was building Arrow fields with no metadata map. CP4D reads field types from Arrow metadata (a serialised CustomFlightAssetField embedded by ArrowConversions.toArrow()). Without it, all columns showed up typeless.
• Fixed by delegating to RestFieldTypeMapper.toAssetField() + ArrowConversions.toArrow() instead of building FieldType directly.
• RestFieldTypeMapper also needed setLength(0) on INTEGER, BIGINT, SMALLINT, TINYINT, and BOOLEAN — without a length, ArrowConversions left those fields indeterminate.
• A separate getFlightInfo() call to Utils.getAssetFields(schema) was overwriting correctly-populated fields with an empty list. Removed.

Liberty gRPC deadlock (CallbackBackpressureStrategy removed)

• Liberty's grpc-1.0 transport never fires onReady(). The strategy loop waited 5 seconds per batch and logged forever. Removed entirely; listener.putNext() is now called directly.

In the previous PR, while solving the hostname issue, a change was made unnoticed that changed the URL creation logic. The host and port should be taken from the connection properties, not the DSL. This logic has been restored in this PR, without affecting the fix for the previous hostname problem.

Test coverage — 2 new test classes + 12 new tests in existing classes; all 45 tests passing.

[piotr-rachwalik]

Can we have some tests for the host/port override change?