feat: add ClientService configuration option

[SMillerDev]

• Resolves: NONE

Summary

This will allow apps to request a client with a base configuration to match their needs. Making the service more widely usable.

TODO

• implement the configuration
• add tests

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)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[come-nc]

Which options are supported? If we merge something like that it will need proper documentation.

[SMillerDev]

In practice it would support whatever the GuzzleClient supports. Do you suggest documenting all the options for it?

[come-nc]

The point of this wrapper is to hide away Guzzle from apps so that they have a stable API they can use.
I’m not knowledgeable enough to understand whether these options are needed and which ones would be useful.

But either we list here options which are supported and we’ll have to work on keep supporting them (meaning even if Guzzle drops them), or we decide that it’s okay to rely on guzzle for this and we add a link to guzzle documentation to see what’s supported.

[SMillerDev]

For news it's these headers and config values: https://github.com/nextcloud/news/blob/master/lib/Config/FetcherConfig.php#L131-L146

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