Update license to Apache 2.0

[pditommaso]

The project should be licensed under Apache 2.0. As such the following license boilerplate should be included in the project root:

https://www.apache.org/licenses/LICENSE-2.0.txt

Moreover, all source files should include this header:

/*
 * Copyright 2023, Seqera Labs
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */