pkg/alertmanager: add URL validation for Webhook receiver

[tomlin7]

Description

Adds URL validation for Webhook receiver configuration fields when loaded from secrets. This ensures URLs are validated regardless of whether configurations come from CustomResources or secrets.

Validated fields:

• url

Relates to #8193

Type of change

What type of changes does your code introduce to the Prometheus operator? Put an x in the box that apply.

• CHANGE (fix or feature that would cause existing functionality to not work as expected)
• FEATURE (non-breaking change which adds functionality)
• BUGFIX (non-breaking change which fixes an issue)
• ENHANCEMENT (non-breaking change which improves existing functionality)
• NONE (if none of the other choices apply. Example, tooling, build system, CI, docs, etc.)

Verification

• Added test case for invalid URLs (Test invalid url returns error)
• Added test case for valid URLs (Test valid url passes validation)
• Fixed existing test that used invalid URL (Test url takes precedence now uses http://example.com/foo)
• All tests pass

Changelog entry

Please put a one-line changelog entry below. This will be copied to the changelog file during the release process.

Add URL validation for Webhook receiver secrets

[heliapb]

lgtm