attrib: Move GIOChannel watcher to g_attrib_register

This patch moves the GIOChannel watcher from g_attrib_new to
g_attrib_register allowing to re-use the same GAttrib functions in the
new Attribute Server code. g_attrib_register function shall be
deprecated when the legacy Attribute Server code gets removed.
gatt: Support ATT read request

This patch adds support for handling ATT read request PDUs.
squash:
gatt: Add Read By Group Type

This patch adds the initial ATT Read By Group Type operation. This ATT
operation is used by the GATT operation to Discover All Primary
Services.