Let Vault modify dynamodb tables and use pay-per-request billing mode.

[michael-diggin]

Fixes Issue #25114.

This PR introduces support for creating the DynamoDB table that Vault would use with PAY_PER_REQUEST billing mode, and introduces support for allowing Vault to modify the billing mode and read/write capacity of the table if it already exists. This is done with new environment variables, and the current default behaviour is left unchanged (no-ops if the table already exists).

[hashicorp-cla]

All committers have signed the CLA.

[schavis]

Content LGTM. Feel free to merge once ENG signs off on the code changes