Fix/add missing itoa ultoa

[jaenrig-ifx]

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CONTRIBUTING.md also tells you what to expect in the PR process.

• Added missing integer to char functions (not provided by the standard library). The file is taken from the renesas Arduino core.
• Added String() constructor examples.

[ederjc]

the license will not be detected by GitHub like this. Can we find a different solution? Second license file in the respective folder?

[jaenrig-ifx]

I still need to test on target. If that works I will open it from the draft.
But this is the reason why this is not compiling:
https://github.com/Infineon/arduino-radar-bgt60/actions/runs/16027236523/job/45218247412?pr=5
And it is not by default provided by Arduino-CoreAPI:
arduino/ArduinoCore-nRF528x-mbedos#6

[jaenrig-ifx]

we find a different solution? Second license file in the respective folder?

Anything below the LICENSE will make GitHub not determine automatically the LICENSE you mean?
Let´s research a bit how, I keep that out of this PR. I would prefer not to maintain bunch of LICENSE files per directory.