[Proposal] mDNS Support

[KazWolfe]

Currently, people using XL-Authenticator need to manually set the IP address of their computer in the app, which requires machines have a (relatively) static IP, require users know their IP address, and only really supports a single instance of XL.

XIVLauncher should instead leverage mDNS/Bonjour to advertise the existence of an _xivlauncher service, which provides an HTTP API for interaction with the launcher. This would allow clients/mobile apps to scan the network and autodiscover instances of the launcher rather than requiring manual user configuration.

The following API endpoints should be available:

• GET /totp/status - HTTP request to check if XIVLauncher is currently waiting for a TOTP code. Clients may poll this endpoint once every second as part of discovery. This endpoint may include information such as SQEX username to allow auto-selection of the proper MFA credential.
• POST /totp/submit - HTTP request to submit a TOTP endpoint for a specific SQEX login request.

As part of this change, the checkbox for "Enable XL Authenticator App" can instead change to "Enable XIVLauncher API." Future additions to the mDNS-based HTTP API may be added at a later point (patch status notifications?).

This service is a candidate for #1433 as well.