feat!: Implement Requirement 1.1.3

[maxveldink]

This PR

Only the last four commits will be merged in with this PR. The others are waiting on #77 and #78.

• Breaking: Renamed API.provider=(provider) to API.set_provider(provider, domain: nil) to better adhere to the specification. I decided to move away from the attr_writer naming convention as the domain parameter would be surprising. I would love others' thoughts on this.
• Provider instances are now stored inside a provider hash in the config instance, with the default provider using nil as the key. The other option I tried here was to store @default_provider in addition to the hash, but this approach was simpler to the slight detriment of readability.
• Refactored API.provider to receive domain as a keyword argument. If no domain is provided, it will return the default provider. This is backward compatible.
• Implemented Requirement 1.1.3 from spec version 0.7.0.

Follow-up Tasks

In subsequent PRs, I'll continue on the flag evaluation specifications.

How to test

Open a console and play around with the set_provider method on the API. If no name is provided, the default provider will be replaced. If a name is provided, the provider will be registered using that name. If the same name is provided, the provider will be overridden.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.39%. Comparing base (25680a4) to head (14b7791).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #80   +/-   ##
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  Coverage   99.39%   99.39%           
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  Files          12       12           
  Lines         164      166    +2     
=======================================
+ Hits          163      165    +2     
  Misses          1        1           

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[maxveldink]

@open-feature/sdk-ruby-maintainers Would love to get this merged in soon as well if anyone has some time for a review!

[beeme1mr]

I updated the PR title to mark it as a breaking change.