Lazy load Open Subtitles credentials

When the Login credentials setting is set to Save in the system keychain version 0.9.5 accesses the system keychain when a video is loaded, not waiting until the credentials are actually needed to log into Open Subtitles. This is behavior documented in the plugin's settings:

If you store your login credentials in the system keychain, you may need to authenticate with your system password when IINA starts. If you choose not to save your login credentials, you will need to enter your opensubtitles.com password every time.

This is particularly undesirable behavior for developers as every time IINA is rebuilt macOS will again prompt for credentials:

This can be avoided by setting Login credentials to Do not save, but of course that has the disadvantage of needing to reenter your Open Subtitles credentials each time when using this feature, which unlike the password for accessing the keychain is usually not a memory item.

Changing the plugin to wait to load the Open Subtitles credentials until they are actually needed would allow developers to keep their Open Subtitles credentials in the system keychain and only be prompted by macOS when actually using Open Subtitles.

The issue names no source files or tests; start by tracing the plugin's settings and Open Subtitles login flow to find where system-keychain credentials are loaded during video loading. Done means keychain authentication is deferred until Open Subtitles is actually used, while the documented credential options continue to work.