Mark ConfigExecTimeout as optional

Problem: when running gitlab-runner list with [runners.custom] section that has no config_exec_timeout field defined, GitLab Runner throws the following warning:

There might be a problem with your config based on jsonschema annotations in common/config.go (experimental feature):
jsonschema: '/runners/0/custom/config_exec_timeout' does not validate with https://gitlab.com/gitlab-org/gitlab-runner/common/config#/$ref/properties/runners/items/$ref/properties/custom/$ref/properties/config_exec_timeout/type: expected integer, but got null

However, GitLab Runner can actually handle this value being empty, see https://gitlab.com/gitlab-org/gitlab-runner/-/blob/fa52677c2bba605c4971a6d61ddabc66b1690b3d/executors/custom/config.go#L14-16.

Solution: mark ConfigExecTimeout as omitempty, similarly to how PrepareExecTimeout and CleanupExecTimeout are marked currently.