feat: add new `WithPollOpts` client option

[jooola]

• Improve consistency with the pattern used with WithRetryOpts,
• Use the PollOpts struct, which is more future-proof than function arguments (properties can be added without breaking),
• Update docs to use the latest WithPollOpts instead of the deprecated alternative.

Since we already changed the way to configure the retry options, I think it is a good time to also change the pattern for the poll opts, even if this does not really bring a new feature.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 71.47%. Comparing base (cf9fdaf) to head (d3b3b05).

Files	Patch %	Lines
hcloud/client.go	25.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #493 +/- ## ========================================== - Coverage 71.53% 71.47% -0.06% ========================================== Files 46 46 Lines 3910 3916 +6 ========================================== + Hits 2797 2799 +2 - Misses 699 704 +5 + Partials 414 413 -1 ```

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