fix: calendar subscription memory exhaustion

[SebastianKrupinski]

• Resolves: # Client Ticket

Summary

• Refactored subscription sync to address memory exhaustion issue

Issue

• Calendar subscription was building the entire calendar import file several times. Building an ics file in memory of around 8/10k of events exhausts all memory.
• Calendar subscription was loading all existing calendar object in to memory to compare etags

Solution

• Don't build the entire calendar in memory, use the import stream reader to read one event at a time.
• Don't load all existing events with data when all we need is the etag

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

[SebastianKrupinski]

@tcitworld @ChristophWurst

We good with the rest of the code?

[ChristophWurst]

Looks good

3rdparty change is probably unrelated

[SebastianKrupinski]

Looks good

3rdparty change is probably unrelated

Correct the 3rdparty is not related, I'll remove it

[tcitworld]

Did not test, but code looks good

[SebastianKrupinski]

Yay, now for the back port fun

[SebastianKrupinski]

/backport to stable32

[SebastianKrupinski]

/backport to stable31

[backportbot]

The backport to stable31 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable31
git pull origin stable31

# Create the new backport branch
git checkout -b backport/56784/stable31

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 1a0535aa

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/56784/stable31

Error: Failed to check for changes with origin/stable31: No changes found in backport branch

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.