Set "REQUIRED" field behaviour flag for fields in minder protos

[eleftherias]

For clients using the openAPI spec, it would be useful to know which input fields are required and which output fields are non-nullable.

We can use the REQUIRED field_behaviour flag to annotate these types of fields.

For example by setting the behaviour to REQUIRED, the typescipt generated field is non-optional:

message Provider {
    // name is the name of the provider.
    string name = 1 [
        (google.api.field_behavior) = REQUIRED
    ];

produces the following typescipt type (note that it does not have a ? marking it as optional):

type v1Provider = {
    /**
     * name is the name of the provider.
     */
    name: string;
}

This task involves finding all the fields that are both required as input AND non-nullable as output.

⚠️ Note that we reuse several messages as inputs and outputs.
We can only set the REQUIRED flag on fields that are both required on input AND never null on output.
If a field is required on input, but may be null on output, we cannot use this flag.