[Features] resource constraints of cluster

[ruijun2002]

According to the feedback as #3637 , instance class restriction on cluster spec has removed. However, in order to prevent the request resources too large or too small to affect the stability of the database, we retain the resource constraints, which is looser than the instance class constraints.
These default resource constraints enable operator to manage the availability and scalability of database clusters. The default constraints are usually sufficient for most use cases, but if your requirements exceed the default limit, you can ignore the constraints in cluster annotation.
For a database cluster, the resource constraints are as below:

• cpu: min=0.5core, max=64core, step=0.5core
• memory：min=cpu * 1Gi * 0.5，max=cpu * 1Gi * 32
• Storage：min=20GiB, max=10TiB
  For example, you can create a mysql cluster with 1 cpu core and memory in any value from 0.5Gi to 32Gi.
  If the number of resources exceeds the specified limit during cluster creation or update, the system prompts user with resource constraints.