Move the `memray attach` callback into a module

[godlygeek]

Previously we were maintaining this as a string literal inside the client-side attach code, but it's growing more complex, and we're reaching the point where it would be advantageous to have it in a normal module that linters and type checkers can analyze.

This commit is based on the code in #455, and will need to be rebased once we land that, but it's ready for review other than that. Marking it draft until #455 is landed.

[codecov-commenter]

Codecov Report

Patch coverage: 80.28% and project coverage change: -0.11% :warning:

Comparison is base (bb9c0db) 91.88% compared to head (d47422d) 91.77%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #459 +/- ## ========================================== - Coverage 91.88% 91.77% -0.11% ========================================== Files 90 92 +2 Lines 10766 10822 +56 Branches 1473 1486 +13 ========================================== + Hits 9892 9932 +40 - Misses 872 888 +16 Partials 2 2 ``` | [Flag](https://app.codecov.io/gh/bloomberg/memray/pull/459/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg) | Coverage Δ | | |---|---|---| | [cpp](https://app.codecov.io/gh/bloomberg/memray/pull/459/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg) | `84.99% <ø> (-0.25%)` | :arrow_down: | | [python_and_cython](https://app.codecov.io/gh/bloomberg/memray/pull/459/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg) | `95.23% <80.28%> (-0.07%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/bloomberg/memray/pull/459?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg) | Coverage Δ | | |---|---|---| | [src/memray/\_attach\_callback.py](https://app.codecov.io/gh/bloomberg/memray/pull/459?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg#diff-c3JjL21lbXJheS9fYXR0YWNoX2NhbGxiYWNrLnB5) | `22.22% <22.22%> (ø)` | | | [src/memray/commands/attach.py](https://app.codecov.io/gh/bloomberg/memray/pull/459?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg#diff-c3JjL21lbXJheS9jb21tYW5kcy9hdHRhY2gucHk=) | `59.41% <100.00%> (+5.47%)` | :arrow_up: | | [tests/integration/test\_attach.py](https://app.codecov.io/gh/bloomberg/memray/pull/459?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg#diff-dGVzdHMvaW50ZWdyYXRpb24vdGVzdF9hdHRhY2gucHk=) | `91.22% <100.00%> (+5.11%)` | :arrow_up: | | [tests/unit/test\_attach.py](https://app.codecov.io/gh/bloomberg/memray/pull/459?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg#diff-dGVzdHMvdW5pdC90ZXN0X2F0dGFjaC5weQ==) | `100.00% <100.00%> (ø)` | | ... and [5 files with indirect coverage changes](https://app.codecov.io/gh/bloomberg/memray/pull/459/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[godlygeek]

Actually... I've realized this will break using a new version memray to attach to a Python process where an older version of memray is installed. Now, that's not really something we've ever officially supported, but it is something that has always mostly worked - the live TUI wouldn't work if the client and server use a different version of our record format, but I believe attaching a tracker that outputs to a capture file has always worked, and that's something that our users might be relying on without us realizing it. That's a strong argument for sticking with having the client inject the code to be executed, rather than having the server import it from its installed copy of memray.

[godlygeek]

Let's not do this. It doesn't work as nicely as I thought it would.