Adds --fast-provide-root and --fast-provide-wait flags to `ipfs dag import`,
mirroring the fast-provide functionality available in `ipfs add`.
Changes:
- Add --fast-provide-root and --fast-provide-wait flags to dag import command
- Implement fast-provide logic for all root CIDs in imported CAR files
- Works even when --pin-roots=false (strategy checked internally)
- Share ExecuteFastProvide implementation between add and dag import
- Move ExecuteFastProvide to cmdenv package to avoid import cycles
- Add logging when fast-provide is disabled
- Conditional error handling: return error when wait=true, warn when wait=false
- Update config docs to mention both ipfs add and ipfs dag import
- Update changelog to use "provide" terminology and include dag import examples
- Add comprehensive test coverage (TestDagImportFastProvide with 6 test cases)
The fast-provide feature allows immediate DHT announcement of root CIDs
for faster content discovery, bypassing the regular background queue.
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