API functions shouldn't be mutating the optional headers= vector.

[NHDaly]

HTTP.jl mutates the supplied headers= vector in-place, to avoid allocations. But that optimization probably does not make sense at the scale of our API calls, and it's a surprising default behavior.

We should change this to manually copy the user-provided headers vector, before passing it to HTTP.jl:

julia> h = ["hello" => "world"]
1-element Vector{Pair{String, String}}:
 "hello" => "world"

julia> rsp = exec(ctx, "nhd-empty-4", "nhd-s", "", headers = h)
RAI.TransactionResponse({
                        "id": "f919be0f-6011-1b54-178e-56cf6468e538",
   "response_format_version": "2.0.4",
                     "state": "COMPLETED"
}, RAI.relationalai.protocol.MetadataInfo(RAI.relationalai.protocol.RelationMetadata[]), Union{}[], Pair{String, Arrow.Table}[])

julia> h
5-element Vector{Pair{String, String}}:
         "hello" => "world"
        "Accept" => "application/json"
  "Content-Type" => "application/json"
    "User-Agent" => "rai-sdk-julia/0.2.1"

[billscheidel-rai]

Note: This issue has been migrated to https://relationalai.atlassian.net/browse/RAI-7063.

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