Added Mnemonic Interface

[dhruv-1001]

Description

This PR adds interface Mnemonic which will make the API to generate new DescriptorSecretKey have type-safe arguments.

Notes to the reviewers

This PR doesn't have any issue linked to it, as it was discusses on a call during the implementation of DescriptorSecretKey (PR #154). It was discussed to make Mnemonic an interface and use that instead of string Mnemonic so that the API to generate DescriptorSecretKey doesn't have any potential failure (like in case it's provided with incorrect Mnemonic words).

APIs added

// generates and returns Mnemonic with random entropy
Mnemonic::new(word_count: WordCount) -> Self { ... } 
// converts string Mnemonic to Mnemonic type with error (in case of incorrect string Mnemonic)
Mnemonic::from_str(mnemonic: String) -> Result<Self, BdkError> { ... }
// generates and returns Mnemonic with given entropy
Mnemonic::from_entropy(entropy: Vec<u8>) -> Result<Self, BdkError> {...}
// view mnemonic as string
Mnemonic::as_string(&self) -> String { ... }

Along with some changes to DescriptorSecretKey::new() to fit these new APIs

Changelog notice

- Added Struct Mnemonic with following methods [#219]
  - new(word_count: WordCount) generates and returns Mnemonic with random entropy
  - from_str(mnemonic: String) converts string Mnemonic to Mnemonic type with error
  - from_entropy(entropy: Vec<u8>) generates and returns Mnemonic with given entropy
  - as_string() view Mnemonic as string
- API removed [#219]
  - generate_mnemonic(word_count: WordCount)

[#219](https://github.com/bitcoindevkit/bdk-ffi/pull/219)

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
  • Top level function generate_mnemonic(...) was removed

[notmandatory]

This looks good, can you also add a Mnemonic.from_entropy() constructor? See also #218 (comment).

[dhruv-1001]

Added the Mnenomic.from_entropy() constructor

[notmandatory]

I think this one is ready to go. I tested it by running also a quick test in test.kts below and all works as expected. We don't need to make any permanent change to test.kts but this was helpful for me to confirm that everything is OK without the Mutex.

import org.bitcoindevkit.*

val network = Network.TESTNET

val mnemonic = Mnemonic(WordCount.WORDS12)

println("mnemonic = ${mnemonic.asString()}")

val descriptorSecKey = DescriptorSecretKey(network, mnemonic, null)

println("descriptorSecKey = ${descriptorSecKey.asString()}")

println("mnemonic = ${mnemonic.asString()}")

[notmandatory]

Also since this is calling bdk code that's already tested I don't think you need to add any addition testing for these new bindings APIs.

[thunderbiscuit]

Just to be clear the "changes" I'm requesting are regarding the documentation. The mnemonic generation is a bit weird to me but I don't want to block this PR on it. Just hoping to start a discussion.

Also: hey glad to see you back @dhruv-1001! It's awesome to see a Summer of Bitcoin student contributing again after the summer is over! 🚀 🚀

[thunderbiscuit]

ACK 9866649.

[notmandatory]

ACK 9866649