INTER-2195: Ship built-in ClientInterface mock using testify/mock

[kevinmia]

Adds a fingerprinttest package that gives SDK users a ready-made mock for testing their own code. The mock is built on testify/mock, a widely-used Go testing library, making it familiar and easy to read for most Go developers. The mock is backed by ClientInterface, a new interface extracted from the existing Client, which makes it injectable via the new WithClientInterface option.

Changes

• fingerprinttest package: new companion module providing a pre-built MockClient for testing code that depends on the Fingerprint SDK
• Added README.md for fingerprinttest and added a usage guide
• Added a CHANGELOG.md for fingerprinttest
• Updated README.mustache by adding Testing section that references fingerprinttest
• Updated go.work: added fingerprinttest to the workspace

Validation

• Created personal unit tests and confirmed it runs successfully. Refer to the `Testing` section in fingerprinttest README for an example of a test

[necipallef]

README template needs to be updated

[mcnulty-fp]

Looks good! Just one nit about the wording in the changelog.

[github-actions]

🚀 Following releases will be created using changesets from this PR:

go-sdk@8.4.0

Minor Changes

• client-interface: Add ClientInterface and WithClientInterface option, allowing custom implementations of the Fingerprint client without needing to implement the low-level API internals required by WithFingerprintAPI. WithFingerprintAPI is now deprecated in favor of WithClientInterface. (5321de3)

Confirmed with @necipallef that this can be dismissed.