[feature/security-first] Add capability control to allow password removal independent of password enforcement

[felix-schwarz]

Description

• OCCapabilities: add support for block_password_removal extension of password policy capabilities

A password policy block with that extension would look like:

"password": {
    "enforced_for": {
        "read_only": true,
        "read_write": true,
        "read_write_delete": true,
        "upload_only": true
    },
    "block_password_removal": {
        "read_only": false,
        "read_write": false,
        "read_write_delete": false,
        "upload_only": false
    },
    "enforced": false
},

To fulfill requirements (owncloud/ios-app#1429), the SDK returns a default value of false for all block_password_removal values not found in a server's capabilities.

Related Issue

owncloud/ios-app#1429

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)