Add support for RDS

[LMJW]

Issue #, if available:

Did not find a particular issue for supporting RDS, but I guess this is probably planned. If RDS support is not needed at the moment, I will close this pull request.

Description of changes:

• clone the smithy model json for rds from GO SDK https://github.com/aws/aws-sdk-go-v2/blob/main/codegen/sdk-codegen/aws-models/rds.2014-10-31.json
• create a "helloword example" for RDS

Note, there is another rds-data smithy model. I will work on to add it if this PR is okay to merge.

Test:

• ./gradlew :aws:sdk:assemble SUCCESS
• ./gradlew :aws:sdk:test SUCCESS
• ./gradlew :aws:sdk:cargoCheck SUCCESS

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[rcoh]

couple of small things, but looks good. Thanks!

[rcoh]

Suggested change

	# used only for static endpoint configuration:
	http = "0.2.3"

(unused here)

[LMJW]

Yes. I will remove this dependency

[rcoh]

can you rename that file to just rds to match the other files?

[LMJW]

Yeah, I will update rds.2014-10-31.json tobe rds.json

[rcoh]

Fixes aws-sdk-rust#36

[LMJW]

Hi @rcoh Please let me know if you have other comments. :)

[rcoh]

LGTM! Can you please just confirm that you ran the example and it worked? (and at least 1 instance was printed). If you're not setup to test that easily just let me know and I can test on my computer before merging.

[LMJW]

Yeah, it worked. Let me try recreate an instance and get the output quickly.

[LMJW]

The example print out is

     Running `/Users/xxx/rust/smithy-rs/aws/sdk/build/aws-sdk/target/debug/rds-helloworld`
DB instance identifier: "test-m5"
DB instance class: "db.m5.large"
DB instance engine: "postgres"
DB instance status: "available"
DB instance endpoint: Endpoint { address: Some("test-m5.c6h6hf3yffse.us-east-1.rds.amazonaws.com"), port: 5432, hosted_zone_id: Some("Z2Rxxxxx") }