chore: abstract plain text settings into a SpannerOption

[sagnghos]

Introduce a SpannerOption usePlainText: a single, additive option that configures the channel for plaintext transport, disables credentials, and sets the protocol for plain-text communication when missing, making it easy and ergonomic to connect to experimental hosts over plain text communication.

[gemini-code-assist]

Summary of Changes

Hello @sagnghos, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant quality-of-life improvement for developers working with experimental Spanner hosts. By abstracting the complex setup for plaintext communication into a single usePlainText() option within SpannerOptions, it simplifies the process of configuring the client to connect to non-TLS endpoints. This change ensures that credentials are automatically disabled and the host URL is correctly formatted for plaintext, making it more ergonomic and secure for experimental use cases.

Highlights

• New usePlainText Option: Introduced a usePlainText() method in SpannerOptions.Builder to simplify configuring plaintext transport for experimental hosts.
• Automated Plaintext Configuration: This new option automatically sets the channel to use plaintext, disables credentials, and ensures the experimental host URL is prefixed with "http://" if it doesn't already specify a protocol.
• Refactored Experimental Host Setup: The ExperimentalHostHelper now leverages the new usePlainText() builder method, centralizing plaintext configuration logic and reducing redundancy.
• Improved Experimental Host Detection: The internal logic for detecting an experimental host was updated from a boolean flag (isExperimentalHost) to checking if the experimentalHost string is non-null, providing a more robust check.
• New Unit Tests: Added comprehensive unit tests to validate the correct behavior of the usePlainText option, covering various host input formats and different orders of builder method calls.

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[gemini-code-assist]

Code Review

This pull request introduces a new usePlainText option in SpannerOptions.Builder to simplify connecting to experimental hosts over plaintext. The implementation correctly handles setting up the channel for plaintext, disabling credentials, and ensuring the host string is correctly formatted with an http:// prefix regardless of the order of method calls. The changes are well-tested. I've added one suggestion to refactor some duplicated logic to improve code maintainability.

[olavloite]

nit: You might want to do a slightly more elaborate check here, considering that this is intended to make things as easy as possible for users, and check that there is also no other protocol prefix present (e.g. add something like Preconditions.checkArgument(!host.startsWith("https:", "Only plain text commmunication is supported for experimental host. Please remove the 'https:' protocol prefix from the host string.")

[sagnghos]

Thanks. I did add a check but in the message mentioned that Please remove the 'https:' protocol prefix from the host string when using plain text communication because we do support mTLS and tls as well using Java Client