Generalise the timeout parameter in actions

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I have a pre-release action that takes some time (`ant` on a GWT project), and it sometimes triggers the Symfony Process component’s 60 s timeout. I’m thinking of making a PR to add a `timeout` option to the `command` action, but before I start working, I should ask:

* Is this desired?
* Should I add it to the `composer-stability-check` and `composer-json-check` prerequisites as well? They use the `executeCommandInProcess` method of `BaseAction`, that uses the Process component;
* Should I also set the default timeout to a higher value, to avoid having to set it on all potential long-running commands in the `rmt.yml` file?
* Would a global timeout option at the root level of the `rmt.yml` file be necessary?

Thanks for making RMT, I love it.

Start by inspecting the command action and BaseAction::executeCommandInProcess, then trace how the Symfony Process component's timeout is configured for command and prerequisite actions. Clarify with maintainers whether timeout should be per action, global in rmt.yml, or both; the work is done when the supported scope and default behavior are agreed and covered consistently.

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