Maintenance: allow e2e test GitHub Action to run on

[dreamorosi]

Description of the feature request

Problem statement

At the moment, every time a contributor opens a PR does that using a fork. The current implementation of the workflow that runs the e2e tests prevents us to run the tests from a fork because it allows us to select only branches of this repo.

In order to allow maintainers to run integration tests on PRs that come from the community, the workflow should be modified.

Summary of the feature

Similar to what done for the "Measure Package Size" workflow, the e2e test workflow should accept an optional parameter that represents a PR Number. This parameter could then be used to checkout the corresponding branch & run the e2e tests.

Note: this action should be performed by maintainers only after having thoroughly reviewed the contribution.

Code examples

See "Measure Package Size" workflow linked above.

Benefits for you and the wider AWS community
Contributors to this project could avoid having to run the e2e tests in their own accounts and potentially incurring in costs. On the maintainers' side instead, this change would result in less manual overhead (see alternatives considered below).

Describe alternatives you've considered
Continue operating like now, and require maintainers to checkout the branch locally, run the e2e tests, and then report the result on the PR.

Additional context
N/A

Related issues, RFCs

N/A

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