Avoid extending Type from Base on Julia 1.12

[ararslan]

The internal Flatbuf module defines a function called Type. On Julia 1.12, defining a new function by defining methods is insufficient to distinguish it from defining new constructor methods for a visible type of the same name. It's now necessary to forward-declare the function to ensure it takes precedence over the other visible binding. Without doing so, Base.Type is extended in Flatbuf instead.

On current main with Julia v1.13.0-DEV.122:

julia> using Arrow
Info Given Arrow was explicitly requested, output will be shown live
WARNING: Constructor for type "Type" was extended in `Flatbuf` without explicit qualification or import.
  NOTE: Assumed "Type" refers to `Base.Type`. This behavior is deprecated and may differ in future versions.
  NOTE: This behavior may have differed in Julia versions prior to 1.12.
  Hint: If you intended to create a new generic function of the same name, use `function Type end`.
  Hint: To silence the warning, qualify `Type` as `Base.Type` in the method signature or explicitly `import Base: Type`.
Precompiling Arrow finished.
  1 dependency successfully precompiled in 4 seconds. 47 already precompiled.
  1 dependency had output during precompilation:
┌ Arrow
│  [Output was shown above]
└

julia> methods(Type)
# 2 methods for type constructor:
 [1] Type(b::UInt8)
     @ Arrow.Flatbuf ~/Projects/julia-packages/arrow-julia/src/metadata/Schema.jl:489
 [2] Type(::Type{T}) where T
     @ Arrow.Flatbuf ~/Projects/julia-packages/arrow-julia/src/metadata/Schema.jl:519

julia> Arrow.Flatbuf.Type === Type
[ Warning: Type is defined in Core and is not public in Arrow.Flatbuf
true

With this PR, the output is the same because of JuliaLang/julia#57546 but will have the correct behavior once that issue is fixed.