Add RDS snapshot quota checks

[kedoodle]

Adds quota checks for Manual DB cluster snapshots and Manual DB instance snapshots https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_Limits.html#RDS_Limits.Limits

$ aws-quota-checker check rds_db_snapshots,rds_db_cluster_snapshots
AWS profile: default | AWS region: ap-southeast-2 | Active checks: rds_db_cluster_snapshots,rds_db_snapshots
Collecting checks  [####################################]  100%
Manual DB cluster snapshots per Region [xxx/ap-southeast-2]: 0/200 ✓
Manual DB instance snapshots per Region [xxx/ap-southeast-2]: 152/200 ✓

[brennerm]

@kedoodle Thanks again for your PR! Looking good

Do you mind renaming the check keys as follows? rds_db_snapshots -> rds_instance_snapshots rds_db_cluster_snapshots -> rds_cluster_snapshots

[kedoodle]

Hey @brennerm,

Appreciate the acceptance of these PRs! I have updated the keys as suggested.

aws-quota-checker check rds_instance_snapshots,rds_cluster_snapshots                   
AWS profile: default | AWS region: ap-southeast-2 | Active checks: rds_cluster_snapshots,rds_instance_snapshots
Manual DB cluster snapshots per Region [xxx/ap-southeast-2]: 0/100 ✓
Manual DB instance snapshots per Region [xxx/ap-southeast-2]: 31/100 ✓

I wasn't able to figure out a single naming convention for the keys. Many have _count suffixes, some have _per_*, and some are just plurals. I like the plurals, as in this PR:

• _count seems redundant in the context of the whole purpose of aws_quota_checker
• _per_x seems to be captured in description, scope, and/or instance ID

[brennerm]

@kedoodle Agree, will align them in the next major version. Thanks again for your support!

[brennerm]

Change has been released in version 1.12.0.