src/shared/att: Add "Find By Type Value" request and response.

This patch adds support for the "Find By Type Value" request and
response with their corresponding parameter structures.
unit/test-att: Add unit test for "Find By Type Value" request/response.

Added test cases for the "Find By Type Value" request/response.
tools/btatt: Add "find-by-type-value" command.

Added command for the "Find By Type Value" request. Sample runs:

$ btatt -d 00:02:5B:00:15:10 -v find-by-type-value 0x0001 0xffff
att: ATT command 0x06
att: < 06 01 00 ff ff 00 28
att: > 01 06 01 00 0a
Error response:
	opcode: 0x06
	handle: 0x0001
	error: 0x0a

$ btatt -d 00:02:5B:00:15:10 -v find-by-type-value 0x0001 0xffff 01 18
att: ATT command 0x06
att: < 06 01 00 ff ff 00 28 01 08
att: > 07 08 00 08 00
Find By Type Value response:
	Handle Information List:
		Attr Handle: 0x0008, Grp End Handle: 0x0008