Add more precise types

[stof]

Description

This adds more precise types for some parts of the package. This PR specifically targets only places that are not generated by the GenerateEndpoints script (this will be covered by a separate PR solving #217).
I found those by running phpstan at level 6 (instead of the existing config using level 5). This PR does not solve all non-endpoints errors of level 6:

• I excluded most of the errors reported in tests (more precise types in tests don't bring benefits to consumers of the package, especially when the missing types are the void return types on test methods themselves)
• I haven't added the more precise type for SerializerInterface::deserialize as I'm not sure about the correct type:
  • $body has a native type ?string but the phpdoc type says string and implementations don't seem to account for null (so maybe string is the expected type
  • I'm not sure about the expected type for $headers as callers are passing $response['transfer_stats'] there (while ringphp documents that response headers are in $response['headers'])
• I haven't solve all errors in Connection because dealing with RingPhp responses is complex.

Those can be solved later (before enabling level 6)

Issues Resolved

n/a

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[dblock]

This is great.

Regarding RingHTTP I would not waste our time. I started removing it in #219, might not have time to get back to it, but it needs to go anyway.