docs(changeset): Add section on version compatibility

[ValuedMammal]

Description

Partially addresses #234 by improving the documentation of ChangeSet by adding a version compatibility policy, clarifying what upgrade/downgrade paths are supported and how breaking changes should be handled. It also adds the #[serde(default)] attribute to locked_outpoints which is a newly added field in 3.0.

In summary the policy says:

• Changes to the ChangeSet data structure will correspond to a major version bump.
• Version N can read/deserialize data written by N-1, i.e., one major version back, but this guarantee doesn't extend to version N-2 or earlier.
• New fields introduced in version N must implement Default so that when deserializing, any missing fields are filled in with the default.
• Version N-1 can deserialize version N data by ignoring unknown fields, with the caveat that not all features from version N will be available.
• Outline a 3-version deprecation cycle

Changelog notice

Changed

• Added #[serde(default)] to ChangeSet::locked_outpoints

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.06%. Comparing base (c422104) to head (80c962b).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #391      +/-   ##
==========================================
- Coverage   79.17%   79.06%   -0.11%     
==========================================
  Files          24       24              
  Lines        5311     5298      -13     
  Branches      242      241       -1     
==========================================
- Hits         4205     4189      -16     
- Misses       1029     1032       +3     
  Partials       77       77              

Flag	Coverage Δ
rust	79.06% <ø> (-0.11%)	⬇️

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[tnull]

Thank you, I much appreciate this move to clearly define the provided guarantees!

[tnull]

Are there any plans to add {backward,forward} compatibility tests in BDK CI to ensure the above described rules are actually enforced at all times?

[oleonardolima]

ACK 137ba7f

[notmandatory]

ACK 137ba7f

[ValuedMammal]

• Updated link to https://doc.rust-lang.org/cargo/reference/semver.html

[luisschwab]

ACK 80c962b

[oleonardolima]

ACK 80c962b

[thunderbiscuit]

Added this item as something to outline in the Release Guide for 3.0. See the current list here: bitcoindevkit/book-of-bdk#137.