[prometheus-thanos] - Allow bucket web httpPort specification

[gjtempleton]

What this PR does / why we need it:

As of version 0.9 there was a rationalisation of startup of HTTP components of Thanos. This meant that the bucket web component moved from being served on port 8080 to 10902 as with the other HTTP components. (See thanos-io/thanos#1687). This isn't strictly necessary to still port-forward to the container as long as users correctly figure out the port however could lead to usability issues and issues with other tooling.

Which issue this PR fixes

N/A

Special notes for your reviewer:

Checklist

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• DCO signed
• Chart Version bumped (if the pr is an update to an existing chart)
• Variables are documented in the README.md
• Title of the PR starts with chart name (e.g. [fluentd-elasticsearch])