feat: MainCamera and VirtualCamera components (#214)

proto/decentraland/sdk/components/common/camera_transition.proto

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+syntax = "proto3";
+package decentraland.sdk.components.common;
+
+message CameraTransition {
+  optional uint32 from_entity = 1;
+  optional uint32 to_entity = 2;
+  oneof transition_mode {
+    float time = 3;
+    float speed = 4; // meters per second; e.g. speed 1 -> 1 meter per second
+  }
+  //   EasingFunction easing_function = 5; // enable in the future
+}

proto/decentraland/sdk/components/main_camera.proto

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+syntax = "proto3";
+package decentraland.sdk.components;
+
+import "decentraland/sdk/components/common/id.proto";
+option (common.ecs_component_id) = 1075;
+
+// PBMainCamera.virtualCameraEntity defines which VirtualCamera entity is active at the moment. 0 means none.
+message PBMainCamera {
+  uint32 virtual_camera_entity = 1; // currently active virtual camera (default: 0)
+//  repeated common.CameraTransition transitionOverrides = 2; // enable in the future
+}

proto/decentraland/sdk/components/virtual_camera.proto

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+syntax = "proto3";
+package decentraland.sdk.components;
+
+import "decentraland/sdk/components/common/camera_transition.proto";
+import "decentraland/sdk/components/common/id.proto";
+option (common.ecs_component_id) = 1076;
+
+// PBVirtualCamera represents a camera to be used at some point in time during the scene execution
+// * The defaultTransition represents the transition TOWARDS this camera.
+// * The lookAtEntity defines to which entity the Camera has to look at constantly (independent from 
+// the holding entity transform).
+message PBVirtualCamera {
+  common.CameraTransition default_transition = 1;
+  optional uint32 look_at_entity = 2;
+}