Allow users to set a global upstream zone size in NginxProxy resource

[kate-osborn]

Add upstream zone size field to the NginxProxy resource so that users can configure a global upstream zone size for NGF.
This zone size will apply to all http and stream upstreams but can be overridden by the zone size in a UpstreamsSettingPolicy.

[mpstefan]

Put this one in the backlog until we see a need to apply this setting globally.

We also will likely want a policy specifically for streams such as TLSRoute when that use case becomes more popular.