feat: viz: add option to add labels to web service

[omer2500]

Problem
There is no option to add labels to the dashboard service (web) specifically
for example, the use case of service mirror the dashboard service to make it accessible in another cluster needs the label:

mirror.linkerd.io/exported: 'true'

Solution
Add option to add labels to dashboard service only

Validation
Run helm templates with couple of use cases to verify it adds new properties while not overwrite others.

I agree to the DCO for all the commits in this PR.

[omer2500]

Hi,

Will be great to get feedback about this :)

thanks