This adds new types and helpers to support the 'initialize' request with
the new typed RequestHandler. While working on this I found there were a
few cases where we incorrectly treated initialize arguments as
capabilities. The new `lldb_dap::protocol::InitializeRequestArguments`
and `lldb_dap::protocol::Capabilities` uncovered the inconsistencies.
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Co-authored-by: Jonas Devlieghere <jonas@devlieghere.com>
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