[Task] Rename SecretKey to PrivateKey

[cycraig]

Description

Replace all usages of the term SecretKey with PrivateKey, and "secret key" with "private key" where appropriate.

Motivation

It was raised on Discord that "secret key" usually refers to symmetric cryptography, while we use it in an asymmetric context, with PublicKey and SecretKey part of KeyPair. Thus to avoid confusion it should probably be renamed.

Renaming it would align with the nomenclature and the term PivateKey is more often used than SecretKey in other projects in the organisation. There doesn't seem to be a particular reason to keep it as-is other than to avoid a breaking change, which is not a major concern as identity.rs is pre-1.0.

This is a low-priority issue, just placing it here to avoid forgetting it.

To-do list

Create a task-specific to-do list . Please link PRs that match the To-do list item behind the item after it has been submitted.

• Rename SecretKey to PrivateKey
• Update comments, documentation

Change checklist

• The feature or fix is implemented in Rust and across all bindings whereas possible.
• The feature or fix has sufficient testing coverage
• All tests and examples build and run locally as expected
• Every piece of code has been document according to the documentation guidelines.
• If conceptual documentation (mdbook) and examples highlighting the feature exist, they are properly updated.
• If the feature is not currently documented, a documentation task Issue has been opened to address this.