Support generating reference resolvers targeting multiple types

### What problem are you facing?

It is currently not possible to generate `ResolveReferences` methods that target multiple suitable types even though there are cases in which this would be desirable.

A good example for this are references to [KMS CMKs](https://doc.crds.dev/github.com/crossplane/provider-aws/kms.aws.crossplane.io/Key/v1alpha1@v0.24.1) and [Aliases](https://doc.crds.dev/github.com/crossplane/provider-aws/kms.aws.crossplane.io/Alias/v1alpha1@v0.24.1) in AWS.

Either can be used as reference in a [SQS Queue's](https://doc.crds.dev/github.com/crossplane/provider-aws/sqs.aws.crossplane.io/Queue/v1beta1@v0.24.1) `kmsMasterKeyId` and Crossplane should support both.

### How could Crossplane help solve your problem?

Resolving of references to KMS Keys for Queues [has been implemented](https://github.com/crossplane/provider-aws/blob/master/apis/sqs/v1beta1/queue_types.go) in the following way:

```
// KMSMasterKeyID - The ID of an AWS-managed customer master key (CMK)
// ...
// +crossplane:generate:reference:type=github.com/crossplane/provider-aws/apis/kms/v1alpha1.Key
KMSMasterKeyID *string `json:"kmsMasterKeyId,omitempty"`
KMSMasterKeyIDRef *xpv1.Reference `json:"kmsMasterKeyIdRef,omitempty"`
KMSMasterKeyIDSelector *xpv1.Selector `json:"kmsMasterKeyIdSelector,omitempty"`
```

It would be absolutely fantastic if crossplane-tools would support listing multiple suitable types here, similar to:

```
// KMSMasterKeyID - The ID of an AWS-managed customer master key (CMK)
// ...
// +crossplane:generate:reference:type=github.com/crossplane/provider-aws/apis/kms/v1alpha1.Key
// +crossplane:generate:reference:type=github.com/crossplane/provider-aws/apis/kms/v1alpha1.Alias
KMSMasterKeyID *string `json:"kmsMasterKeyId,omitempty"`
KMSMasterKeyIDRef *xpv1.Reference `json:"kmsMasterKeyIdRef,omitempty"`
KMSMasterKeyIDSelector *xpv1.Selector `json:"kmsMasterKeyIdSelector,omitempty"`
```

If there are multiple suitable candidates users would use `matchLabels` and suitable matches to differentiate between these resources.

Thank you!