feat!: expose chainposition first and last seen timestamps

[reez]

Description

Exposes upstream ChainPosition::Unconfirmed first_seen and last_seen fields instead of collapsing them into a single timestamp.

Breaking: ChainPosition::Unconfirmed.timestamp is replaced by first_seen and last_seen. The previous timestamp value corresponds to last_seen.

Notes to the reviewers

Follow-up from #1032 review discussion.

Documentation

• bdk_wallet
• bitcoin
• uniffi
• Other:
  • bdk_chain::ChainPosition
  • bdk_chain source

Changelog

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added exactly one changelog:* label
• I've linked the relevant upstream docs or specs above

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[ItoroD]

ACK 15ab034