arrow

GH-35134: [C++] Add `arrow_vendored` namespace around double-conversion library (#35135)

### Rationale for this change

See #35134

This change avoids potential symbol collisions when linking Arrow within other libraries.

This actually seems like the original intention (a private namespace) per the following merge: 767c953
The message in that merge is confusing, because it implies that the merge made the library private, when it actually did the opposite (specifically in 3b89b19). A quote is below:

>  Also, make its use private, because of Windows DLL exports. 

### What changes are included in this PR?

* an `update.sh` script to 
  * automatically pull a new version, 
  * apply the namespace change, 
  * apply a custom patch for seemingly Gandiva (15137e2)
    * this patch will break in future versions of double-conversion, but it is at least documented in the update script
* the update script is run for the current version 

The script is based on: https://github.com/apache/arrow/blob/main/cpp/src/arrow/vendored/fast_float/update.sh

### Are these changes tested?

Yes. Built and tests run

### Are there any user-facing changes?

Only if users used to depend on using the internal vendored double-conversion libraries for their own code. In such a case, when they upgrade, they would have to specify the `arrow_vendored` namespace. 
* Closes: #35134

Authored-by: Mike Lui <mikelui@meta.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>