feat: add WITHIN search keys

[paulnivin]

Adds RFC 5032 WITHIN support behind the ext_within feature gate.

[coveralls]

Coverage Report for CI Build 33269123002

Coverage increased (+0.06%) to 91.489%

Details

• Coverage increased (+0.06%) from the base build.
• Patch coverage: 71 of 71 lines across 5 files are fully covered (100%).
• 9 coverage regressions across 1 file.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

9 previously-covered lines in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
imap-types/src/arbitrary.rs	9	76.54%

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Coverage Stats

Relevant Lines:	11749
Covered Lines:	10749
Line Coverage:	91.49%
Coverage Strength:	914.28 hits per line

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💛 - Coveralls

[duesee]

Thank you (again) for this easy to review and well done PR :-)

Ah, and... welcome as a contributor!