digitalocean: add API rate limit

[tja]

Description

This change adds a new command line parameter, --digitalocean-api-rate-limit, which allows to throttle calls to the DigitalOcean API. The DO API has a rate limit of 250 requests per minute, which is consistently hit when managing 100+ (sub-) domains.

Fixes #4572

Checklist

• Unit tests updated
• End user documentation updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: tja / name: Dr. Thomas Jansen (ab6194b)

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[tja]

Hi @mloiseleur and @szuecs, would you mind giving me an estimate when to expect the review. Thanks in advance!

[mloiseleur]

Hi @tja,

FYI, we are currently moving providers out of tree, using webhook. See #4347
Infoblox provider has already moved, see #4516

[tja]

@mloiseleur, that's actually pretty cool! I'm unsure what that means for my PR, though. Does that mean it's better to wait or would integrating it before the move would make more sense to you?

[mloiseleur]

@tja It means you can either:

1. Submit a PR to provide an this Rate Limit parameters for all providers (not only Digital Ocean)
2. Create a webhook provider for Digital Ocean based from current in-tree code and add this feature to it

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