Do not store binary data inside php files

[mikhainin]

Follow up on #10404, the initial description:

For the sake of performance, the hex2bin call was removed from the generated PHP code in #8006
However, after this PR all autogenerated files contain binary data, which makes it hard or even impossible to see using common tools (git and github included (i.e. this file), as well as some code editors) since most software considers such files as binary ones and not as code.

Alexander suggested using a hex representation of string literals and I updated the original patch a bit, not hex-encoding printable characters.

Benchmarks from #10404#issuecomment-1635939062

The percent of printable chars: 0.83869 (80112 vs 15408)
hex parsing: 3.22371, length=382090
hex2Bin parsing: 1.18489, length=191059
hexAscii parsing: 0.89524, length=142306 (suggested option)
binary parsing: 0.26437, length=95542

[haberman]

Some tests are failing, and they look unrelated to your change. Could you merge with main and re-push?

[mikhainin]

Apologies for this. I rebased from main and force-pushed the branch

[mikhainin]

Found one more issue with binary data.

When the repository has settings in .gitattributes:

*.php text eol=lf

git will break protobuf descriptors and will give developers hard debugging times. The same effect will be achieved if git config core.autocrlf is set to true.

Not sure if we need to make it a separate issue or no: it definitely worsens developer experience but probably not directly relates to this PR.

[mikhainin]

@haberman, would it be possible to take a look by any chance?