Add NimBLEAttValue class.

[h2zero]

This is a specialized container class to hold BLE attribute values.

• Removes the use of std::string previously used to store the values.
• Allows for setting/getting/notifying values from std::string, std::vector<uint8_t>, Arduino String, const char*, and uint8_t buffers.
• Has operators retrieve the value as std::string, Arduino String, std::vector<uint8_t>, uint8_t* or char pointers.
• Includes iterators and subscript/random access operator.
• Introduces a max length parameter to the creation of server characteristics/descriptors to limit the size of the memory footprint.
• Nearly Seamless integration with existing code.
• Adds a config option to enable/disable timestamp storage when the value is updated.
• Adds a config option to specify the initial size of the value container if not specified in the constructor.

Examples

Writing POD types to a remote characteristic:

• pRemoteChar->writeValue(10);
• pRemoteChar->writeValue("TEST");
• pRemoteChar->writeValue(23.45);
• pRemoteChar->writeValue('X');

Write container types to a remote characteristic:

• pRemoteChar->writeValue(String("test string");
• pRemoteChar->writeValue(std::string("test std::string");
• pRemoteChar->writeValue(std::vector<uint8_t>({1,2,3,4,5,6}));
• pRemoteChar->writeValue(NimBLEAttValue({1,2,3,4,5,6}));

Reading POD types from a remote characteristic:

• int i_val = pRemoteChar->readValue<int>();
• `double d_val = pRemoteChar->readValue();
• `const uint8_t* u8p = pRemoteChar->readValue();

Reading container types from a remote characteristic:

• std::string ss_str(pRemoteChar->readValue());
• String ar_str = pRemoteChar->readValue();
• std::vector<uint8_t> vec = pRemoteChar->readValue();
• NimBLEAttValue att_val = pRemoteChar->readValue();

Reading a single byte from the value:

uint8_t val = pRemoteChar->readValue()[2];

Printing the hex bytes of a value:

for (auto it :  pRemoteChar->readValue()) {
    printf("value: %02x\n", it);
 }

Note: The server attributes work the same way but with setValue()/getVaue()

Directly using the NimBLEAttValue class:

// create an attribute value with a maximum capacity of 8 bytes, initialized to "TEST" (an extra byte will be allocated for the NULL) 
NimBLEAttValue my_att_val("TEST", 8); 

my_att_val += "1234"; // Value now contains "TEST1234"
printf("my_att_val = %s\n", my_att_val.c_str()); // use the c_str() method, print's "TEST1234"

// Concatenate a String and a NimBLEAttValue (same use for std::string)
String test_str = "my att value is: ";
test_str += my_att_val;
Serial.println(test_str);

[h2zero]

Finally got around to cleaning this up, if anyone would like to test it out please provide feedback. I'd be especially interested in some criticism from any C++ template wizards out there.

[h2zero]

Merging this as I have seen no issues so far in long term testing.