Add secrets policies

[llewellyn-sl]

Add 3 permissions policies for AWS Secrets Manager:

• Tower instance role
• ECS execution role
• ECS IAM trust relationship
• EC2 custom role
• EC2 custom trust policy

These policies are added to align with Tower docs updates in https://github.com/seqeralabs/nf-tower-docs/pull/350

The necessity to delete the existing secrets policies in this repo is to be determined.

[gwright99]

I'm losing track of what's going on. Approving this so the files are all in the same repo. We can take a second pass to figure out if any of the related prose on the Secrets page needs to change afterwards.

[llewellyn-sl]

I'm losing track of what's going on. Approving this so the files are all in the same repo. We can take a second pass to figure out if any of the related prose on the Secrets page needs to change afterwards.

There is no prose in this PR. This is just to add your new secrets policies (related to the secrets updates docs PR in Cloud)