feat(octez): serialise directory

[huancheng-trili]

Context

Part of JSTZ-121; in preparation for #655.

JSTZ-121

Description

Serialise directory.

Manually testing the PR

• Unit test: added new test cases

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.59%. Comparing base (bf1af2d) to head (7d75dc5).
Report is 1 commits behind head on main.

Files with missing lines	Coverage Δ
crates/octez/src/async/directory.rs	97.16% <100.00%> (+1.01%)	⬆️

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