Add exact UID-set fetch API

[mpscholten]

Summary

• add fetchByByteStringSet for fetching arbitrary data items from an exact list of UIDs
• encode sparse UIDs as an IMAP sequence-set such as 101,205,999, rather than widening them to a range
• return immediately for an empty UID list without sending an invalid UID FETCH command
• preserve the existing raw ByteString values and response-UID mapping used by fetchByByteStringR

Why

IMAP searches commonly return sparse UIDs. The existing fetchByByteStringR API accepts only a contiguous range, so using the first and last search result can fetch many unrelated messages between them. Calling the single-UID API in a loop avoids over-fetching but adds one network round trip per message.

This helper allows callers to batch metadata reads such as BODYSTRUCTURE for exactly the search results they need.

Validation

• Nix doCheck build with GHC 9.10.3
• imap-parsers: 50 cases, 0 errors, 0 failures
• Haddock generation
• regression coverage for sparse and empty UID sets