wip: add persisted ADR

docs/adr/0002_persisted.md

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+# Introduce `PersistedWallet`
+
+* Status: accepted
+* Deciders: [list everyone involved in the decision]
+* Date: 2024-08-19
+
+Technical Story: PR [#1547](https://github.com/bitcoindevkit/bdk/pull/1547), [ADR-0001](./0001_persist.md)
+
+## Context and Problem
+
+We would like persistence operations to be both safe and ergonomic. It would cumbersome and error prone if users were expected to ask the wallet for a changeset at the correct time and persist it on their own. Considering that the wallet no longer drives its own database, an ideal interface is to have a method on `Wallet` such as `persist` that the user will call and everything will "just work."
+
+## Decision Drivers
+
+* [driver 1, e.g., a force, facing concern, …]
+* [driver 2, e.g., a force, facing concern, …]
+* … <!-- numbers of drivers can vary -->
+
+## Considered Options
+
+* [option 1]
+* [option 2]
+* [option 3]
+* … <!-- numbers of options can vary -->
+
+## Decision Outcome
+
+Chosen option: Introduce a new type `PersistedWallet` that wraps a BDK `Wallet` and provides a convenient interface for executing persistence operations aided by a `WalletPersister`.
+
+### Positive Consequences <!-- optional -->
+
+* [e.g., improvement of quality attribute satisfaction, follow-up decisions required, …]
+* …
+
+### Negative Consequences <!-- optional -->
+
+* [e.g., compromising quality attribute, follow-up decisions required, …]
+* …
+
+## Pros and Cons of the Options <!-- optional -->
+
+### [option 1]
+
+Open questions
+
+* How does a user know when and how often to call `persist`? (Short answer: whenever we mutate the wallet)
+
+### [option 2]
+
+[example | description | pointer to more information | …] <!-- optional -->
+
+* Good, because [argument a]
+* Good, because [argument b]
+* Bad, because [argument c]
+* … <!-- numbers of pros and cons can vary -->
+
+### [option 3]
+
+[example | description | pointer to more information | …] <!-- optional -->
+
+* Good, because [argument a]
+* Good, because [argument b]
+* Bad, because [argument c]
+* … <!-- numbers of pros and cons can vary -->
+
+## Links <!-- optional -->
+
+* [Link type] [Link to ADR] <!-- example: Refined by [ADR-0005](0005-example.md) -->
+* … <!-- numbers of links can vary -->