ux: add Hubble capabitilies to cilium install and cilium upgrade

[nbusseneau]

Right now, Hubble has to be enabled by cilium hubble enable later than at install time:

It would be great if Hubble could be enabled right from the start with cilium install with an additional flag (--hubble?) and possibility to specify the relay image (--relay-image).

This capability should also be conferred to cilium upgrade: we should detect if Hubble is enabled, and then automatically upgrade the relay image along the agent and operator images (and also provide --relay-image for users to specify a specific image).