Stop the timeline from requesting back pagination whilst loading existing items.

[pixlwave]

This PR makes 3 changes (and is probably easiest to review commit by commit):

• Detached timelines were erroring due to attempting to subscribe to back pagination status (apparently no longer supported by the SDK).
• Make sure we await the loading of existing timeline items now that Rust doesn't provide them to us when setting up the subscription - fixes a bug where an error would be shown when navigating to an event permalink.
• Even once the above was fixed, the TimelineTableViewController could sometimes attempt to back paginate whilst loading the existing items. Now we use an initial pagination state that will prevent this from happening until the items have loaded.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions[bot]]

	Warnings
:warning:	Some of the commits are missing ticket numbers. Please consider squashing all commits that don't have a tracking number.
:warning:	You seem to have made changes to views. Please consider adding screenshots.

Generated by :no_entry_sign: Danger Swift against 964fec0cc568ea8c2aba2ba32e1e7995fa80659a

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 14.28571% with 12 lines in your changes missing coverage. Please review.

Project coverage is 77.99%. Comparing base (342cfc0) to head (964fec0).

Files	Patch %	Lines
...urces/Services/Timeline/RoomTimelineProvider.swift	0.00%	12 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #2969 +/- ## =========================================== - Coverage 78.01% 77.99% -0.02% =========================================== Files 693 693 Lines 54164 54175 +11 =========================================== - Hits 42254 42253 -1 - Misses 11910 11922 +12 ``` | [Flag](https://app.codecov.io/gh/element-hq/element-x-ios/pull/2969/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=element-hq) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/element-hq/element-x-ios/pull/2969/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=element-hq) | `68.73% <14.28%> (-0.09%)` | :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=element-hq#carryforward-flags-in-the-pull-request-comment) to find out more.

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