Feat: add custom sts endpoint configuration field for various aws features in kong gateway

[windmgc]

Description

This is a new feature in kong gateway 3.8. The original PR is here: https://github.com/Kong/kong-ee/pull/9654

The feature covers the update for three parts of the AWS-related features inside Kong gateway: the aws-lambda plugin, the aws vault backend, and the IAM authentication for RDS database.

KAG-4599

Testing instructions

Preview link:

Checklist

• Review label added
• Conditional version tags added, if applicable.

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[windmgc]

There are two things that I did not update in this PR: one is the new fields in kong.conf page, another is the aws-lambda plugin's schema page.
I think those two are auto updated after new gateway version release, could you please help verify? @lena-larionova Thank you!

If not then I can update those two manually

[cloudjumpercat]

There are two things that I did not update in this PR: one is the new fields in kong.conf page, another is the aws-lambda plugin's schema page.
I think those two are auto updated after new gateway version release, could you please help verify? @lena-larionova Thank you! If not then I can update those two manually

@windmgc Yep, those are updated automatically and I think Lena's already updated them in the docs site.

[cloudjumpercat]

Looks good to me! Thanks for making this PR!