ephemeral/password: Introduce a new ephemeral password resource

[austinvalle]

This PR is currently in draft as an ephemeral random password is not particularly useful until managed resources are updated to allow interaction with ephemeral data (via write-only attributes, earliest that will be available is Terraform v1.11.0)

This PR introduces a new random_password ephemeral resource which is functionally similar to the managed resource, minus the keepers and id attributes. Once write-only attribute are introduced in Terraform v1.11, this resource can be used to generate a random password while avoiding storing that password in state.

Notes

• keepers are not relevant because ephemeral resources do not produce a plan and are never stored in state. Triggers for producing an ephemeral password will eventually be the responsibility of the module author.
• id is not relevant because the new testing framework doesn't require this information and the practitioner doesn't benefit from the duplication with result.

Remaining TODOs on this PR

• Changelogs
• Update documentation + examples to indicate a sample use-case (i.e. generate random password, pass to AWS DB instance write-only attribute)