Allow additional data

Every implementation I've seen of the BCF API has additional data in the responses; it's only natural, since every software has features not found in others that they want to handle.

However, I think it's possible to standardize these infos (at least some of them), so that a client could at least display them, and even allow to dynamically generate a form with values. There's already some semblance of this in the `files_information` endpoint but I think it's not enough.

Here's a quick example.

## Define data

Create a `/meta` route suffix for some routes: projects, topics, files, comments, documents. I feel these are probably the most likely to have non-standardized informations.

- `/bcf/{version}/projects/meta`
- `/bcf/{version}/projects/{project_id}/topics/meta`
- `/bcf/{version}/projects/{project_id}/topics/{guid}/files/meta`
- `/bcf/{version}/projects/{project_id}/topics/{guid}/comments/meta`
- `/bcf/{version}/projects/{project_id}/documents/meta`

These routes would return the following informations:

```json
[
{
"code": "X",
"label": "Label X",
"id": "f8fab089-e26a-46d1-8a6c-721e4a8435d8",
"is_mandatory": false,
"is_modifiable": true,
"maximum_length": 200,
"value_type": "String"
},
{
"code": "Y",
"label": "Label Y",
"id": "9e203f1b-7881-4cda-8dac-acc706184595",
"is_mandatory": false,
"ismodifiable": true,
"maximum_length": 10,
"value_type": "List",
"values": [
{
"code": "A",
"label": "Label A"
},
{
"code": "B",
"label": "Label B"
}
]
}
]
```

## Display data

The data defined in the `/meta` endpoint would be returned in the following way, for instance for a project list `/bcf/{version}/projects/`:

````json
[{
"project_id": "F445F4F2-4D02-4B2A-B612-5E456BEF9137",
"name": "Example project 1",
"additional_data": [
{ "code": "X", "value": "Lorem ispum" },
{ "code": "Y", "value": "A" }
]
}, {
"project_id": "A233FBB2-3A3B-EFF4-C123-DE22ABC8414",
"name": "Example project 2",
"additional_data": [
{ "code": "X", "value": "Donec tortor nisi" },
{ "code": "Y", "value": "B" }
]
}]
````