feat: Add string concatenation for secrets

[ravishankar15]

PR Description

Adds string concatenation for the secrets type

Which issue(s) this PR fixes

Fixes #1514

Note to reviewers

I am not sure why we have secrets and optionalsecrets. I believe secrets was the older implementation. Hence i skipped adding this functionality on the secrets type. Let me know if we have to add this to the secrets type it is easily extendable.

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated

[wildum]

Thanks for your contribution, I think that this is the right approach and it works well for optional secrets.
We do still have values of the type "Secret", for example the remote.value component will export secrets and not optional secrets. It would be nice to also have concatenation for secrets and not only optional secrets. optional secret + secret = secret and string + secret = secret.
The type casts and switch nesting make the code hard to follow and I'm afraid that it might become even harder with the secret extension. Maybe you can cast the types at the beginning of the map function:

lhsOptSecret, lhsIsOptSecret := lhs.Interface().(alloytypes.OptionalSecret)
lhsSecret, lhsIsSecret := lhs.Interface().(alloytypes.Secret)
etc...

That's just an idea, if you find a better way please go ahead.
One last little thing: can you update the doc here with Concatenate strings/secrets

[wildum]

I would prefer a more precise name like the one before, maybe "tryUnwrapSecrets"

[ravishankar15]

I see your thought about nesting getting complicated. I wanted to make this implementation little generic so that it can be extended to any type of capsules in future. I have split the handling to separate functions with comments. Let me know if this looks over engineered and we wanted to stick to handling only secrets for now.