JSON-based DDS options

[maloel]

High-level changes:

• disable Viewer auto update of read-only options, now relying on options-changed notifications
• following last PR, Viewer now gets range/read-only even if option isn't supported
• renamed dds_option_type value from "number" to "integer"; removed rs2_option_value::as_unsigned
• options_watcher now sends a notification when a value cannot be queried (i.e., the value becoming null is a change, too)

DDS options:

• Now have several types: float, int, string, IPv4 (boolean, choices still missing)
• librealsense query() returns the cached value from DDS options
• Options can now be read-only or have optional (null-able) values
• Option range is in the base option class, but optional
  • At first I had "float" and then "ranged-float" classes, and same for "ranged-int", but this complicated the code needlessly
• Options enforce value type but not range or read-only
  • So options serve to communicate as metadata rather for logic enforcement
• dds-adapter can now send options even without a value, as optional

TODO:

• DDS options still cannot change from non-read-only to read-only (during streaming, for example)