feat(sdk/go): add kubernetes authentication provider

[GeorgeMac]

This adds a new AuthenticationProvider which automatically configures the client to use our Kubernetes authentication method.
With this provider configured, the client automatically authenticates using the Kubernetes method, by exchanging the service account token on the host with the target Flipt instance. It uses the default service account token path, which can be overridden. While the previously exchanged client token exists and is valid (not expired) within some configurable leeway, it uses the previously exchanged client token.
When the token draws close to expiry it exchanged it automatically for a new one. It does so by re-reading the service account token and exchanging with Flipt again. This is important, because Kubernetes will update the contents of that file, as its own service account tokens are automatically refreshed.

This PR additionally introduces k8s auth method to the matrix of methods used in the integration tests.
Doing so allows us to ensure the Go SDK works across the entire API with this authentication method.
To achieve this, there is a little thin JWKS provider implementation added that supports using its own CA.
This is to mimic how Kubernetes runs its own OIDC discovery mechanisms in-cluster.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (51a3d17) 73.02% compared to head (ce529fe) 73.02%.

Additional details and impacted files

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##             main    #2703   +/-   ##
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  Coverage   73.02%   73.02%           
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  Files          85       85           
  Lines        8370     8370           
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  Hits         6112     6112           
  Misses       1880     1880           
  Partials      378      378           

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[markphelps]

looking great!

[GeorgeMac]

Something funny going on in actions land. The env var we generate for use in naming the upload artefacts is occassionally going missing 🤔 . Some builds it gets generated appropriately, sometimes not. I think something funny is going on in the actions environment.

[GeorgeMac]

OK, I see this only appears to happen when the test itself fails because it steps over the env var setting.
Im going to reorder steps so this doesn't confuse in the future.

[yquansah]

Looks great!

[markphelps]

looks great! mind updating the docs as well?