Provide default values for mirror config options

[flyinghyrax]

(...finally.)

This should be a much more focused PR for addressing issue #1702 without so much additional refactoring.

I've changed the configuration files embedded in the package like so:

• Renamed the existing "default.conf" to "example.conf". This file is still used when the -c argument is passed file that doesn't exist to create a configuration.
• Added "defaults.conf", containing only default values for all [mirror] configuration options except for directory, with minimal annotation.

I changed BandersnatchConfig:

• It's now a subclass of ConfigParser. Technically this violates the Liskov substitution principal by giving the subtype a different signature for __init__. I have mixed feelings about that, but this seemed like a much cleaner way to customize ConfigParser and removed many instances of BandersnatchConfig().config or similar.
• It always reads the embedded defaults file unless explicitly disabled via a keyword argument (useful for some existing unit tests). The user config file is read second if specified, so values from the user config overwrite values from the defaults file. This leaves the configuration singleton with default values set for any mirror option the user doesn't configure.

Some notable effects of this include:

• It is no longer meaningful to use ConfigParser.has_option on the 'mirror' section. Instead, you have to check whether the options value is empty or None.
• Specifying a default/fallback value when calling .get on the 'mirror' section will have no effect, because the option will already be present in the ConfigParser mappings.

Happy to tweak which options are required/have defaults and dynamic fallback behaviors for things like root_uri, if that's something we want to change at this point.

Fixes #1702 Fixes #990

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.83333% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 83.69%. Comparing base (4d020e8) to head (74986a2). Report is 96 commits behind head on main.

Files	Patch %	Lines
src/bandersnatch/configuration.py	80.00%	11 Missing and 1 partial :warning:
src/bandersnatch/main.py	61.53%	4 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1740 +/- ## ========================================== + Coverage 79.69% 83.69% +4.00% ========================================== Files 31 33 +2 Lines 4324 4380 +56 Branches 780 783 +3 ========================================== + Hits 3446 3666 +220 + Misses 721 531 -190 - Partials 157 183 +26 ```

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