fix: allow custom URL input for Seafile remotes (and others where relevant)

[ivanskodje]

Solves #380

Problem

Editable dropdown fields are hardcoded to false! This means in order to add and use our own self-hosted seafile data, we need hacky solutions, like manually editing conf files.

Solution

Make it conditional based on rclone's exclusive metadata flag, which indicates whether or not custom values are allowed.

The app should already parse this flag from the rclone config providers JSON into ProviderOption.exclusive

Since Seafile's URL option does not set Exclusive: true in rclone's source, custom URLs will now be permitted.

Example of how to verifying the rclone config Seafile uses (Debian/Ubuntu)

# Install rclone if you haven't already
sudo apt install rclone -y

# Extract relevant URL config from seafile
rclone config providers | jq '.[] | select(.Name == "seafile") | .Options[] | select(.Name == "url")'

Note there's no Exclusive field (defaults to false):

{
  "Name": "url",
  "Help": "URL of seafile host to connect to",
  "Provider": "",
  "Default": "",
  "Value": null,
  "Examples": [
    {
      "Value": "https://cloud.seafile.com/",
      "Help": "Connect to cloud.seafile.com",
      "Provider": ""
    }
  ],
  "ShortOpt": "",
  "Hide": 0,
  "Required": true,
  "IsPassword": false,
  "NoPrefix": false,
  "Advanced": false,
  "DefaultStr": "",
  "ValueStr": "",
  "Type": "string"
}

You can also verify that we parse this Exclusive field in ProviderOption.kt and set it to item.exclusive.

Testing / Verify

Add a new Seafile remote and verify that:

1. The default URL appears as a dropdown choice
2. You can enter a custom URL