feat(rds): add standard connection secret keys for crossplane-runtime compatibility

[Keralin]

Summary

Add username and password keys to aws_rds_cluster connection secrets to enable compatibility with provider-sql and other consumers that expect standard crossplane-runtime connection secret keys.

Changes:

• config/cluster/rds: Add username (alias of master_username) and password (from master_password) to connection secrets
• config/namespaced/rds: Same changes for namespaced variant

Existing keys (master_username) are preserved for backwards compatibility.

Connection Secret Output (after this fix)

data:
  endpoint: xxxxxx
  reader_endpoint: xxxxxx
  master_username: xxxxxx  # Preserved for backwards compatibility
  username: xxxxxx         # NEW - Standard crossplane-runtime key
  port: xxxxxx
  password: xxxxxx         # NEW - Standard crossplane-runtime key

Fixes #1189

Related issues:

• MySQL Instance fills out endpoint field in ConnectionSecret in an unexpected way, breaking crossplane-contrib/sql-provider #778
• Agreement on how to write connection parameters #1578
• Adds port twice when using connectionStrings from upbound/provider-aws-rds provider-sql#191
• Agreement on how to write connection parameters provider-sql#203

[github-actions]

This provider repo does not have enough maintainers to address every pull request. Since there has been no activity in the last 90 days it is now marked as stale. It will be closed in 14 days if no further activity occurs. Leaving a comment starting with /fresh will mark this issue as not stale.

[Keralin]

/fresh

[Upbound-CLA]

All committers have signed the CLA.