fix: add a policy to allow services to access global SSM parameters

[lgarvey]

Addresses .

This PR adds an additional policy to the service task role so that services can retrieve secrets tagged with Copilot-application = '__all__'.

This enables services to read SSM parameters that are global/account wide such as the VPC EGRESS_IP SSM parameter, and the prometheus /observability/prometheus/adot_config parameters.

There are two associated PRs to add these tags to global SSM params:
uktrade/terraform-platform-modules#201
uktrade/terraform-module-aws_account#74

A platform-documentation PR is required - it'll be raised before merging this PR.

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Checklist:

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Tasks:

• Trigger the pull request regression tests for this branch and confirm that they are passing