[API] Make Tenant namespace-scoped

[claudiolor]

Description

This PR changes the API to make the Tenant resource namespace-scoped.
The reasons for this are the following:

• When the tenant namespace is deleted, all the resources related to the tenant can be deleted
• When creating peering connection, changes can be limited only to the tenant-namespace

Moreover, a webhook has been added to make sure that the Tenant resource is created in a Tenant namespace with matching cluster ID and to make sure that the Tenant has unique name across the cluster.

How Has This Been Tested?

Added test suite for the new webhook.
Manual and automated e2e tests have been performed.

[adamjensenbot]

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[claudiolor]

/build

[claudiolor]

/build

[claudiolor]

/test

[fra98]

/test

[fra98]

/merge