feat: Initial OpenFgaClientConfig

[jimmyjames]

Initial support for autoconfiguration of an OpenFgaClient.

Description

Creates an openFgaClient bean from configuration.

Not all configurations available via Configuration are added in this PR (e.g., maxRetries, timeouts). We can capture another issue to see what other configurations should be surfaced here.

I also added some very brief info to the README; obviously will need more work, but wanted to capture some of the details.

Some details about how the OpenFgaClient is configured given property values:

• If no credentials properties are specified, no credentials (no authorization) is configured for the client
• If a credentials.api-token property is specified, that (and only that) will be used to configure the client
• If other credentials properties are specifed, they will be used to configure the client

Closes #2

References

https://github.com/openfga/java-sdk/tree/main?tab=readme-ov-file#initializing-the-api-client

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

[rhamzeh]

Q: We are copying the enum and validation here, any chance we can reuse what's in the SDK?