Support multiple shared pools

[Levovar]

Is your feature request related to a problem? Please describe.
Right now only pool can be of type shared. Shared pool in itself represents a medium-level contract: nothing is guaranteed as with an exclusive pool. but an app can be still sure there won't be any overcommitting, and that context switches will tried to be kept to the minimum.
In a multi-tenant environment it can be imagined a namespace would like to get this level of commitment, but would like to only share a specific set within its own namespace.

Supporting this would also enable us to create a separate shared pool for infra services, this isolating them from other app shared, and the default pool.

Describe the solution you'd like
The extra check inside the DP safeguarding against this config needs to be removed.
The way of how the SHARED_CPUS environment variable is provisioned into the app container must be changed.
Alternatively, we could also consider entirely removing this environment variable, considering we don't pint on share resources anymore.