Make scram secret assocation optional

[dougbw]

what

• Make the aws_msk_scram_secret_association resource optional when sasl/scram authentication is enabled through a new variable client_sasl_scram_secret_association_enabled
• The default behaviour is the same as before

why

• The aws_msk_scram_secret_association resource is not very flexible and in certain use cases the preference is to manage the secret associations out of band from the cluster creation
• In our case we need to enable sasl/scram at the point of creating the cluster of the cluster, but need to manage sasl/scram users in another tf root
• Having aws_msk_scram_secret_association resources in separate roots creates conflicts where they will constantly overwrite each others changes

references

• See aws_msk_scram_secret_association overrides/removes existing secret associations hashicorp/terraform-provider-aws#16791

[aknysh]

@dougbw thanks for the PR, LGTM.
Can you please run the following commands and commit the changes:

make init
make github/init
make readme

[aknysh]

/test all

[aknysh]

thanks @dougbw