mesa/extensions: Add extension::version

mesa/extensions: Add extension::version

Enable limiting advertised extension support by context version with
finer granularity. This new field is currently unused and is set to
0 everywhere. When it is used, a value of 0 will indicate that the
extension is supported for any version of a context.

v2: Use uint*t type for version and note the expected values (Emil)
    Use an 8-bit data type
    Reformat macro for better readability (Chad)

v3: Note preparatory nature of commit (Chad)

Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>