v3.0.1: Support for new keystores and binary secrets

The major functional change between v2 and v3 is the addition of synchronous support for the Secret Service via the dbus-secret-service crate. This means that keyring users of the Secret Service no longer need to link with an async runtime.

The main API change between v2 and v3 is the addition of support for non-string (i.e., binary) "password" data. To accommodate this, two changes have been made:

1. There are two new methods on Entry objects: set_secret and get_secret. These are the analogs of set_password and get_password, but instead of taking or returning strings they take or return binary data (byte arrays/vectors).

2. The v2 method delete_password has been renamed delete_credential, both to clarify what's actually being deleted and to emphasize that it doesn't matter whether it's holding a "password" or a "secret".

Another API change between v2 and v3 is that the notion of a default feature set has gone away: you must now specify explicitly which crate-supported keystores you want included (other than the mock keystore, which is always present). So all keyring client developers will need to update their Cargo.toml file to use the new features correctly.

All v2 data is fully forward-compatible with v3 data; there have been no changes at all in that respect.

The MSRV has been moved to 1.75, and all direct dependencies are at their latest stable versions.

(The original 3.0.0 release had a compile-time issue related to Error traits in some environments; v3.0.1 fixes that.)