`Project` owner canonicalization to be moved from conversion to REST strategy

[vpnachev]

How to categorize this issue?
/area quality
/kind enhancement

What would you like to be added:
The Project API has an owner field, in practice the owner subject could be also a project member with additional roles assigned. There is some code canonicalizing the resource, but it is placed in the version <-> internal conversion methods

gardener/pkg/apis/core/v1beta1/conversions.go

Lines 132 to 149 in a3cbecf

	if owner := out.Owner; owner != nil {
	outer:
	for i, member := range out.Members {
	if member.Name == owner.Name && member.APIGroup == owner.APIGroup && member.Kind == owner.Kind {
	// add owner role to the current project's owner if not present
	for _, role := range member.Roles {
	if role == core.ProjectMemberOwner {
	continue outer
	}
	}
	out.Members[i].Roles = append(out.Members[i].Roles, core.ProjectMemberOwner)
	} else {
	// delete owner role from all other members
	out.Members[i].Roles = removeRoleFromRoles(member.Roles, ProjectMemberOwner)
	}
	}
	}

gardener/pkg/apis/core/v1beta1/conversions.go

Lines 159 to 198 in a3cbecf

	if owner := out.Owner; owner != nil {
	outer:
	for i, member := range out.Members {
	if member.Name == owner.Name && member.APIGroup == owner.APIGroup && member.Kind == owner.Kind {
	// add owner role to the current project's owner if not present
	if member.Role == core.ProjectMemberOwner {
	// remove it from owners list if present
	out.Members[i].Roles = removeRoleFromRoles(member.Roles, ProjectMemberOwner)
	continue outer
	}
	for _, role := range member.Roles {
	if role == ProjectMemberOwner {
	continue outer
	}
	}
	if out.Members[i].Role == "" {
	out.Members[i].Role = core.ProjectMemberOwner
	} else {
	out.Members[i].Roles = append(out.Members[i].Roles, core.ProjectMemberOwner)
	}
	} else {
	// delete owner role from all other members
	out.Members[i].Roles = removeRoleFromRoles(member.Roles, ProjectMemberOwner)
	if member.Role == ProjectMemberOwner {
	if len(out.Members[i].Roles) == 0 {
	out.Members[i].Role = ""
	} else {
	out.Members[i].Role = out.Members[i].Roles[0]
	if len(out.Members[i].Roles) > 1 {
	out.Members[i].Roles = out.Members[i].Roles[1:]
	} else {
	out.Members[i].Roles = nil
	}
	}
	}
	}
	}
	}

A better place for such code is the REST strategy, i.e. in the Canonicalize method

gardener/pkg/apiserver/registry/core/project/strategy.go

Line 60 in fb264cf

func (projectStrategy) Canonicalize(_ runtime.Object) {

Why is this needed:
Improve code quality and readability.