SuccinctRoles: add zero padding to bins

Add zero padding to bin names inside SuccinctRoles.
Zero padding ensures that the bin names always have the same length.

This characteristic is implied in the example given by TAP 15 where
the third bin is named "alice.hbd-03". For context read TAP 15:
https://github.com/theupdateframework/taps/blob/master/tap15.md

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

Author: Martin Vrachev

tuf/api/metadata.py

@@ -1443,12 +1443,9 @@ class SuccinctRoles(Role):
     in the graph.
 
     The total number of bins is 2 to the power of the passed ``bit_length``.
-    Targets are assigned to bins by casting the left-most ``bit_length`` of
-    bits of the file path hash digest to int, using it as bin index between 0
-    and ``2**bit_length - 1``.
 
-    Bin names are the concatenation of the passed ``name_prefix`` and a hex
-    representation of the bin index between separated by a hyphen.
+    Bin names are the concatenation of the passed ``name_prefix`` and a
+    zero-padded hex representation of the bin index separated by a hyphen.
 
     The passed ``keyids`` and ``threshold`` is used for each bin, and each bin
     is 'terminating'.
@@ -1485,6 +1482,14 @@ def __init__(
         self.bit_length = bit_length
         self.name_prefix = name_prefix
 
+        # Calculate the suffix_len value based on the total number of bins in
+        # hex. If bit_length = 8 then number_of_bins = 256 or 100 in hex
+        # and suffix_len = 3 meaning the third bin will have a suffix of "003"
+        self.number_of_bins = 2**bit_length
+        # suffix_len is calculated based on "number_of_bins - 1" as the name
+        # of the last bin contains the number "number_of_bins -1" as a suffix.
+        self.suffix_len = len(f"{self.number_of_bins-1:x}")
+
     def __eq__(self, other: Any) -> bool:
         if not isinstance(other, SuccinctRoles):
             return False
@@ -1522,6 +1527,10 @@ def get_role_for_target(self, target_filepath: str) -> str:
         """Calculates the name of the delegated role responsible for
         ``target_filepath``.
 
+        The target at path ``target_filepath`` is assigned to a bin by casting
+        the left-most ``bit_length`` of bits of the file path hash digest to
+        int, using it as bin index between 0 and ``2**bit_length - 1``.
+
         Args:
             target_filepath: URL path to a target file, relative to a base
                 targets URL.
@@ -1535,7 +1544,9 @@ def get_role_for_target(self, target_filepath: str) -> str:
         # bit_length bits that we care about.
         shift_value = 32 - self.bit_length
         bin_number = int.from_bytes(hash_bytes, byteorder="big") >> shift_value
-        return f"{self.name_prefix}-{bin_number}"
+        # Add zero padding if necessary and cast to hex the suffix.
+        suffix = f"{bin_number:0{self.suffix_len}x}"
+        return f"{self.name_prefix}-{suffix}"
 
 
 class Delegations: