Add String Implementation - StatisticsAggregator

[jpdakran]

• Please check if the PR fulfills these requirements

• Tests for the changes have been added

• Docs have been added / updated

• All CI checks are green

• What kind of change does this PR introduce?
  Bug Fix

• What is the current behavior?
  NotImplementedError when calling str on StatisticsAggregator #662
  When calling detect-secrets audit .secrets. baseline --stats we get an error since there is no implementation for the StatisticsAggregator __str__ method.

• What is the new behavior (if this is a feature change)?
  Return a string representation of the statistics. See below for an example

• Does this PR introduce a breaking change?
  No

• Other information:

Plugin: PrivateKeyDetector
Statistics: True Positives: 0, False Positives: 0, Unknown: 8, Precision: 0.0, Recall: 0.0

Plugin: TwilioKeyDetector
Statistics: True Positives: 0, False Positives: 0, Unknown: 1, Precision: 0.0, Recall: 0.0

Plugin: HexHighEntropyString
Statistics: True Positives: 0, False Positives: 0, Unknown: 2, Precision: 0.0, Recall: 0.0

Plugin: KeywordDetector
Statistics: True Positives: 0, False Positives: 0, Unknown: 3, Precision: 0.0, Recall: 0.0

Plugin: JwtTokenDetector
Statistics: True Positives: 0, False Positives: 0, Unknown: 1, Precision: 0.0, Recall: 0.0

Plugin: Base64HighEntropyString
Statistics: True Positives: 0, False Positives: 0, Unknown: 1, Precision: 0.0, Recall: 0.0