Enable Alloy to run with env vars and extra args

[johannaojeling]

To enable Alloy to run as a background service with user-defined environment variables and extra command line arguments, this PR updates the Alloy formula to:

• Create an empty file $(brew --prefix)/etc/alloy/config.env for env vars
• Create an empty file $(brew --prefix)/etc/alloy/extra-args.txt for extra args
• Create an wrapper script $(brew --prefix)/Cellar/alloy/<version>/bin/alloy-wrapper which runs Alloy with the env vars and extra args, if any
• Update the service run command to execute the wrapper script

These changes are motivated by the necessity to:

• Pass the extra argument --stability.level=public-preview to run Alloy with remote config support
• Set certain environment variables, i.e. GCLOUD_RW_API_KEY, GCLOUD_FM_COLLECTOR_ID and GCLOUD_FM_LOG_PATH, for the Alloy self-monitoring pipelines in Grafana Cloud Fleet Management to work

[hairyhenderson]

LGTM! Thanks @johannaojeling!

[mattdurham]

What does this do to existing installations on an update?

[johannaojeling]

What does this do to existing installations on an update?

If you already have installed Alloy with the formula from the master branch, brew upgrade results in no changes as long as the Alloy version is unchanged: Warning: alloy 1.5.1 already installed. Then you will have to explicitly brew reinstall to get the updates.

If you have an earlier version installed, e.g. 1.5.0, brew upgrade will install the newer version using the updated formula.

[mattdurham]

lgtm