manager: fix digest auth for ami/http mechanism.

Due to a fixed size buffer the digest authentication could be
incorrectly calculated if a large URI was provided, causing
authentication failure. The buffer is now dynamically allocated to allow
any size URI within the normal limits of the HTTP request size.

ASTERISK-27841

Change-Id: I660609db13b8f9e5f9567f339dd804f4985d41b3