Implement AdminService.GetDLQTasks (#4884)

<!-- Describe what has changed in this PR -->
**What changed?**
I added `AdminService.GetDLQTasks`, and I implemented it in the
`AdminHandler`.

<!-- Tell your future self why have you made these changes -->
**Why?**
We want to copy these APIs to the admin service so that we can use them
from tdbg.

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test? Checked in staging env? -->
**How did you test it?**
I added a unit test which covers 100% of the new code.

<!-- Assuming the worst case, what can be broken when deploying this
change to production? -->
**Potential risks**
The fact that this code is literally copy-pasting all the protos from
the history service means that we will end up with a lot of redundant
code that does nothing but replace `historyservice` with `adminservice`
and vise-versa.

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sent to the broader community? (Yes/No) -->
**Is hotfix candidate?**