feat: advanced watcher usernames management

[wI2L]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

This CL introduces a new plugin watcher that allows to add new
watcher usernames to a task, similar to how the tag plugin works.

A new field allowed_watcher_usernames has also been added to the
core model of a task template to allows pre-defined watchers usernames
at the template level. The pre-defined list of usernames of the template
is merged with the input list given during the creation of a task, ignoring
any duplicate values.

• What is the current behavior? (You can also link to an open issue here)

n/a

• What is the new behavior (if this is a feature change)?

See description.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No, new feature.

• Other information:

n/a

[ovh-cds]

CDS Report test#798.0 ✘

• Stage 1
  • Test ✘

[wI2L]

Will be adding some tests, but I would like to gather feedback about the implementation first.
EDIT: Need to fix prototype of Exec function of existing plugins as well.

[ovh-cds]

CDS Report test#799.0 ✘

• Stage 1
  • Test ✘