[Task] Improve `Stronghold` constructor and `did_create` impl

[PhilippGackstatter]

Description

After #787 we should make some quality of life and security improvements to our Stronghold wrapper. Some of these will be breaking changes.

• Increase PBKDF2_HMAC_SHA512 iterations to 120,000.
  • Note that this breaks using older stronghold snapshot files, so it can be considered a more severe breaking change than changing an API.
• Implement transaction-like behaviour in did_create. Investigate if this is necessary for other functions as well. This should perhaps also be documented and/or recommended behaviour.
  • For instance, Someone re-trying to create an identity from an existing keypair might fail every time, because the DID was added to the index, but the client syncing later failed.
• Add a create flag for Stronghold that, if true, will create a snapshot file if it doesn't exist, or, if false, throw an error if the file doesn't exist.

This last point warrants re-thinking the Stronghold constructor.

The broad options are (copied from @cycraig):

1. Stronghold::new and Stronghold::new_with_options with a StrongholdOptions (would have to be a builder to avoid breaking changes).

   Stronghold::new_with_options("./snapshot.hodl", "password", StrongholdOptions::new().dropsave(false)).await?;

2. Introducing a StrongholdBuilder, e.g.,

   Stronghold::builder()
     .path("./snapshot.hodl")
     .password("password")
     .dropsave(false)
     .create(false)
     .build()
     .await?;

3. Placing everything in a non-exhaustive StrongholdOptions struct, embracing the TypeScript approach:

   Stronghold::new(StrongholdOptions {
     path: "./snapshot.hold".into(),
     password: "some-password".into(),
     create: false,
     ..Default::default()
   }),await?;

---

I prefer option 1, because the other options leave the possibility of a user not providing a password and path which are required, requiring us to introduce an error variant (à la MissingRequiredField). This can of course be further discussed.

Motivation

Improve security and provide users a way to better express their expectations (create flag).

Resources

• PBKDF2 iterations comment
• Original comment for stronghold constructor changes

Change checklist

Add an x to the boxes that are relevant to your changes, and delete any items that are not.

• 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.