Add support for builds on multiple branches

[xKevin04]

This is an edge case I haven't encountered before, but apparently it's possible for a build to be on multiple branches at the same time (at the very least, the default master and a beta one). This would make the branch property of product.json files an array instead of null/string (with a single element in almost every case). Alternatively, the build could also be listed twice, in a denormalized fashion - just like the API does it.

BROK The InvestiGator currently has version 1.4.8 available via roll back from the master branch and also from a dedicated beta channel named after the version. Build id, published date and everything else is identical, only the branch differs. Response content from https://content-system.gog.com/products/1252694989/os/windows/builds?generation=2 below for posterity:

{
   "total_count": 88,
   "count": 6,
   "items": [
       {
           "build_id": "58928999546942343",
           "product_id": "1252694989",
           "os": "windows",
           "branch": null,
           "version_name": "1.5.3.7",
           "tags": [
               "receiver_v2",
               "csb_11_0_0_w_19",
               "11_0_0_19"
           ],
           "public": true,
           "date_published": "2025-08-26T15:15:14+0000",
           "generation": 2,
           "link": "https:\/\/gog-cdn-fastly.gog.com\/content-system\/v2\/meta\/07\/83\/0783659ae585027d29363cd0b172cb6b"
       },
       {
           "build_id": "58880509110441043",
           "product_id": "1252694989",
           "os": "windows",
           "branch": null,
           "version_name": "1.5.2",
           "tags": [
               "receiver_v2",
               "csb_11_0_0_w_19",
               "11_0_0_19"
           ],
           "public": true,
           "date_published": "2025-08-09T21:45:39+0000",
           "generation": 2,
           "link": "https:\/\/gog-cdn-fastly.gog.com\/content-system\/v2\/meta\/3c\/86\/3c86262277d2dd6cfc3caa7731573b8c"
       },
       {
           "build_id": "58326746111321867",
           "product_id": "1252694989",
           "os": "windows",
           "branch": null,
           "version_name": "1.4.8",
           "tags": [
               "receiver_v2",
               "csb_11_0_0_w_19",
               "11_0_0_19"
           ],
           "public": true,
           "date_published": "2025-01-30T21:37:26+0000",
           "generation": 2,
           "link": "https:\/\/gog-cdn-fastly.gog.com\/content-system\/v2\/meta\/77\/dd\/77ddfb7d3b1e2a5e8e67df650349133f"
       },
       {
           "build_id": "57316509858366998",
           "product_id": "1252694989",
           "os": "windows",
           "branch": null,
           "version_name": "1.4.7",
           "tags": [
               "receiver_v2",
               "editor_v_1_4_0",
               "csb_10_6_1_w_158"
           ],
           "public": true,
           "date_published": "2024-02-17T10:27:12+0000",
           "generation": 2,
           "link": "https:\/\/gog-cdn-fastly.gog.com\/content-system\/v2\/meta\/bd\/56\/bd564ee7b75774470579ad6b0396c774"
       },
       {
           "build_id": "57101968185977286",
           "product_id": "1252694989",
           "os": "windows",
           "branch": null,
           "version_name": "1.4.6.1",
           "tags": [
               "receiver_v2",
               "editor_v_1_4_0",
               "csb_10_6_1_w_158"
           ],
           "public": true,
           "date_published": "2023-12-05T10:19:59+0000",
           "generation": 2,
           "link": "https:\/\/gog-cdn-fastly.gog.com\/content-system\/v2\/meta\/ba\/72\/ba726854a6837eb1721dbe22f20eff63"
       },
       {
           "build_id": "58326746111321867",
           "product_id": "1252694989",
           "os": "windows",
           "branch": "1.4.8",
           "version_name": "1.4.8",
           "tags": [
               "receiver_v2",
               "csb_11_0_0_w_19",
               "11_0_0_19"
           ],
           "public": true,
           "date_published": "2025-01-30T21:37:26+0000",
           "generation": 2,
           "link": "https:\/\/gog-cdn-fastly.gog.com\/content-system\/v2\/meta\/77\/dd\/77ddfb7d3b1e2a5e8e67df650349133f"
       }
   ],
   "has_private_branches": true
}