Add license information in the GTFS feed

### Describe the problem

Many GTFS feeds are published under licenses that might restrict certain aspects of re-use. The information for these license are often found on the agency websites and can be hard to find. This creates ambiguity for use cases of the GTFS, and might delay their ingestion in trip planners.

### Use cases

Adding license information to the GTFS spec allows consumers to easily figure out what they can use the GTFS for, whether they can modify it, re-share it, etc.
For consumers who work with hundreds of GTFS, the operation can even be automatized using the license information in the GTFS.

### Proposed solution

Solution 1) Adding fields to `feed_info.txt` since it can contain non-rider-facing information.

`license_spdx_id`: The [SPDX ID](https://spdx.org/licenses/) of the GTFS feed license
`license_name`: The name of the license
`license_url`: linking to either the website of the license, to SPDX or to a page in the agency website that details the conditions of use of the GTFS.

Solution 2) Adding a file called `license_info.txt` containing `license_spdx_id`, `license_name` and `license_url`.

A drawback of this 2nd solution is that `license.txt` is a popular non-CSV file for software in general.

### Additional information

[GBFS has license information](https://gbfs.org/specification/reference/#system_informationjson). In v3, `license_url` and `license_id` (SPDX ID) are conditionally forbidden. That can be the case with GTFS by providing a `license_id` if it exists in the [SPDX license list](https://spdx.org/licenses/), otherwise prividing a `license_url`.