Add helpers to handle UUID<->str conversion

We add two helper functions here. First one uuid2str converts bt_uuid_t
to a printable string, always on 128-bits format.
The second one do an inverse conversion: string to bt_uuid_t. We have
support to read 16-bits version (eg. 0x1234) and 128-bits.
Add search_svc command

With search_svc command, we can search services on remote device.
If we need a specific service, it's possible to filter by UUID that can
be 16-bits or 128-bits.
Add included command

After we find which services there are at remote device, we can
discover the relationship between them. The included command help us
with this, giving which services are included at specified service ID.
README: Add a limitations section

At limitations section, we can explain some limits that we have on the abtcl.