Add support for "max_execution_time" setting in GW OS UCI

[gbraux]

• I have searched the issues of this repository and believe that this is not a duplicate.

Summary

The default timeout for Javascript custom codecs is fairly low, and may need to be increased depending on the underlying hardware platform. For instance, my Raspberry PI 0W with full ChirpstackOS image often trigger timeouts.

The good news is that a setting exists in chirpstack.toml to tweak timeouts:-)

[codec]
[codec.js]
max_execution_time="1000ms"

The bad news is that this setting has not been implemented (yet) in the GW OS UCI configuration.
This feature request is to implement it. Should be similar to this recent commit that added support to the reset_pin setting in UCI for Concentratord : chirpstack/chirpstack-openwrt-feed@3669968

Current workaround

SSH into the GW OS and add the setting to the static config section of the script generating the toml file. (https://github.com/chirpstack/chirpstack-openwrt-feed/blob/master/chirpstack/chirpstack/files/chirpstack.init). This will not be retained over OS upgrades ...

Can you implement this by yourself and make a pull request?

I have very limited dev skills unfortunately ;-)