feat(elasticache): added elasticache subnetgroup, fixed replicationgroup

[haarchri]

Signed-off-by: haarchri chhaar30@googlemail.com

Description of your changes

• added elasticache subnetgroup
• fixed replicationgroup created (async true) & synced (skip lateinit fields)
• created ref/selector for subnetgroup in replicationgroup and cluster
• added example for elasticache subnetgroup
• added example for elasticache replicationgroup
• changed ec2/subnet to fits us-west-1a

Fixes #195 #196

I have:

• [x] Read and followed Crossplane's contribution process.
• [x] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

NAME                                                                READY   SYNCED   EXTERNAL-NAME         AGE
subnetgroup.elasticache.aws.jet.crossplane.io/example-subnetgroup   True    True     example-subnetgroup   86m

NAME                                                                          READY   SYNCED   EXTERNAL-NAME              AGE
replicationgroup.elasticache.aws.jet.crossplane.io/example-replicationgroup   True    True     example-replicationgroup   23m