Do not add tags to spec when ObserveOnly

[turkenh]

Description of your changes

We should not add default tags to the spec of the managed resources when the management policy is set as ObserveOnly as we are only observing the resource in read-only mode.

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Together with crossplane-contrib/provider-upjet-aws#672

Before this PR:

apiVersion: rds.aws.upbound.io/v1beta1
kind: Instance
metadata:
  ...
spec:
  deletionPolicy: Delete
  forProvider:
    region: us-west-1
    tags:
      crossplane-kind: instance.rds.aws.upbound.io
      crossplane-name: rds-observe-only
      crossplane-providerconfig: default
  managementPolicy: ObserveOnly
  providerConfigRef:
    name: default
  writeConnectionSecretToRef:
    name: example-dbinstance-out
    namespace: default
status:
  atProvider:
    ...

With this PR:

apiVersion: rds.aws.upbound.io/v1beta1
kind: Instance
metadata:
  ...
spec:
  deletionPolicy: Delete
  forProvider:
    region: us-west-1
  managementPolicy: ObserveOnly
  providerConfigRef:
    name: default
  writeConnectionSecretToRef:
    name: example-dbinstance-out
    namespace: default
status:
  atProvider:
    ...

[lsviben]

Thanks @turkenh !

[ulucinar]

Thanks @turkenh, lgtm.