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 79.17%. Comparing base (c422104) to head (137ba7f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #391   +/-   ##
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  Coverage   79.17%   79.17%           
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  Files          24       24           
  Lines        5311     5311           
  Branches      242      242           
=======================================
  Hits         4205     4205           
  Misses       1029     1029           
  Partials       77       77           

Flag	Coverage Δ
rust	79.17% <ø> (ø)

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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

[oleonardolima]

nit: maybe it's a good idea to also refer to: https://doc.rust-lang.org/cargo/reference/semver.html (not a blocker though).