fix: fix for a possible E0401 error in dependent libs

[robert-affinidi]

If the library is referenced in another lib (affinidi-did-resolver-cache-sdk = { version = "0.2.2" }) then the compilation error occurs:

❯ cargo build
   Compiling affinidi-did-resolver-cache-sdk v0.2.2
error[E0401]: can't use `Self` from outer item
   --> /.../affinidi-did-resolver-cache-sdk-0.2.2/src/lib.rs:232:54
    |
226 | impl DIDCacheClient {
    | ---- `Self` type implicitly declared here, by this `impl`
...
232 |     pub async fn new(config: ClientConfig) -> Result<Self, DIDCacheError> {
    |                                                      ^^^^
    |                                                      |
    |                                                      use of `Self` from outer item
    |                                                      refer to the type directly here instead
    |
    = note: a `const` is a separate item from the item that contains it

For more information about this error, try `rustc --explain E0401`.
error: could not compile `affinidi-did-resolver-cache-sdk` (lib) due to 1 previous error