att: Fix coding style

Fix coding style for enc_read_blob(), enc_read_blob_resp(),
dec_read_resp(), enc_error_resp(), enc_find_info_req(),
enc_find_info_resp() and enc_prep_write().
attrib: Removed unused parameter from enc_write_resp()
att: Add decode prepare write support

Add function for decoding Prepare Write Request PDUs.
att: Add encode prepare write support

Add function for encoding Prepare Write Response PDUs.
att: Add decode execute write support

Add function for decoding Execute Write Request PDUs.
att: Add encode execute write support

Add function for encoding Execute Write Response PDUs.
unit: Add test for long write using gatt_write_char()
unit: Add test for gatt_discover_char()
attrib: Run callback with the partial result of characteristic discovery

Callbacks should check for status equal to ATT_ECODE_ATTR_NOT_FOUND to
ensure that the characteristic discovery has finished.
unit: Add test for gatt_discover_char() (by UUID)
attrib: Reduce scope of variable declarations in char_discovered_cb()

This makes it easy to identify where variables are being used.