Add support for network mounts

[mdegat01]

Proposed change

Add support for network mounts to supervisor. Initial support will include cifs and nfs. For cifs users can provide a username and password if auth is used.

When adding a network mount you must say if it is used for backups or media. If used for backups then users will be given an option to create or restore backups from there (to be implemented in subsequent PR). If used for media then the mount will be available as a folder in /media with the same name as the mount (this is implemented in this PR).

On startup supervisor will automatically try and mount all configured mounts. Or if it detects they are already mounted it adopts them. If supervisor cannot mount them for some reason it reports that as a supervisor issue (to be made into a core repair in a follow-up PR to core)

Steps still to do:

1. Backups need to skip the mount folders in /media
2. Changes to mounts need to persist across restarts correctly
3. CLI changes to correspond to API changes
4. Document new APIs
5. Core repair from new MOUNT_FAILED issue

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality to the supervisor)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: Document new APIs around network mounts developers.home-assistant#1762
• Link to cli pull request: Add mounts commands for network share support cli#395

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast supervisor tests)
• Tests have been added to verify that the new code works.

If API endpoints of add-on configuration are added/changed:

• Documentation added/updated for developers.home-assistant.io