Verify relation type in resolve{Relation}Metadata

[bajtos]

Improve helper functions resolveHasManyMetadata, resolveBelongsToMetadata, resolveHasOneMetadata to assert that the provided metadata has the right relation type.

This is important because in some places we are casting generic RelationMetadata to a more specific variant, thus bypassing compiler checks.

See #3387 for more details, here is an example implementation for belongsTo: https://github.com/strongloop/loopback-next/pull/3387/files#diff-ec9fc2ab14e201fb6c5ddb370ba094faR27-R31

Acceptance criteria

For each of the existing relation type, modify resolve{Relation}Metadata to assert that metadata.type is the expected relation type. Include a test to verify.

• BelongsTo
• HasMany
• HasOne