Puts the string value at the end of the bdata_str.
In case the bdata_str is not empty a space is added before the string value. The stored value can be read back
with the util_bdata_get_string.
The strings are encoded as "length-value", quotes for clarity only.
| bdata_str |
a StringBuilder to store a backend specific data (bdata) |
| value |
a value to store |