Support []string and []*string as sensitive fields

[sergenyalcin]

Description of your changes

Fixes #100

This PR provides support for []string and []*string as sensitive fields. In resource manifests, more than one secrets and more than one fields of secrets can be referred as input for sensitive values. Also, sensitive field outputs will be written to the connection secret.

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

I have tested this PR by using the example in the issue description.

• Firstly, I added a custom configuration for aws_elasticache_user that marks the passwords field as sensitive.
• Then I created the following manifest:

apiVersion: elasticache.aws.jet.crossplane.io/v1alpha1
kind: User
metadata:
  name: sample-user
spec:
  forProvider:
    userName: "testUserName"
    accessString: "on ~app::* -@all +@read +@hash +@bitmap +@geo -setbit -bitfield -hset -hsetnx -hmset -hincrby -hincrbyfloat -hdel -bitop -geoadd -georadius -georadiusbymember"
    engine: "REDIS"
    region: us-west-1
    passwordsSecretRef:
      - name: test-slice-1
        namespace: crossplane-system
        key: pwd
      - name: test-slice-2
        namespace: crossplane-system
        key: pwd
  writeConnectionSecretToRef:
    name: test-conn
    namespace: default

• The external resource was successfully created and the connection secret had the correct values.

[turkenh]

Thanks a lot @sergenyalcin for dealing with this.

PR is looking pretty good, left couple of comments mostly asking for some code cleanup.

[turkenh]

Great work, thanks @sergenyalcin 👍