feat: allow setting or unsetting the boto retry configuration

[ryancausey]

feat: allow setting or unsetting the boto retry configuration

This adds the ability to directly set the boto retry configuration
dictionary, or to leave it unset and allow botocore to automatically
discover the configuration from the environment or ~/.aws/config
files.

The default is to use the previous PynamoDB behavior for configuring
retries so as to not make this a breaking change.

Closes #1268

[ryancausey]

@ikonst this is my first attempt at adding this feature. I wanted to check that this was in line with what we discussed in #1268 before polishing and adding tests. Please review and let me know, thanks!

[ryancausey]

The good news is this change works locally for me and confirms that botocore is handling the AWS SDK configuration discovery logic, not boto3. I was able to override the setting to be None and it automatically picked up the retry configuration values in my ~/.aws/config file.

[ikonst]

Let's see where we should document this.

[ryancausey]

I'm going to try and finish adding tests and docs today. If there's a specific place besides the settings docs this should be mentioned, let me know.

[ryancausey]

@ikonst I made some updates and added tests and docs. Please re-review. Please note the changes to Connection.__init__ that allow for direct users of the Connection class to leave the botocore retry configuration unset. I couldn't just pass through None since that has been used to indicate "pull this config value from the settings file".