Werk #5499: Implicit default values for agent updater config

Some fields, namely the update interval and the proxy configuration (new since 1.5) are optional within the agent updater plugin configuration. With this werk, a default value of 3600 seconds for the interval and a default behavior of "ignore proxy settings" is assumed in the case that these fields are not set by the user. This matches the behavior of previous Check_MK releases.

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