Allow templatable service target to support scripts

[frenck]

Proposed change

With #48469 we added support for the use of Selectors in script fields.
This includes the target selector, however, since scripts do not have !input capabilities, it has to rely on templates.

Currently, this doesn't work:

- service: light.turn_on
  target: "{{ my_script_var_field }}"

This PR adjusts that oversight and ensures selectors for scripts in 2021.4.0 work as expected.

⚠️ This is a new feature, but also a bug fix for 2021.4... A bit in the middle 🤷‍♂️ Right now it can cause unexpected behavior (as I already learned on Discord today, hence this PR).

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

• No score or internal
• 🥈 Silver
• 🥇 Gold
• 🏆 Platinum

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.