Add ssl.enabled property for HTTP client settings

[sandeep780049]

Adds the missing `spring.http.clients.ssl.enabled` property (and the matching per-service-client property) to `HttpClientSettingsProperties.Ssl`, aligning HTTP client SSL configuration with every other `*.ssl.enabled` property in Spring Boot.

As noted in gh-51472 (surfaced during review of gh-51468), `HttpClientSettingsProperties.Ssl` only exposed `bundle`. There was no way to explicitly disable SSL when a bundle was inherited from base settings or configured at group level. This PR treats it as a bug of omission per @wilkinsona's direction on the issue:

• `ssl.enabled=true` with no bundle: falls back to the JDK's default trust material (no bundle applied, so existing behavior is preserved)
• `ssl.enabled=false`: acts as a kill switch — clears any SSL bundle, including one inherited via `orElse` from base settings, mirroring the semantics of e.g. `spring.data.redis.ssl.enabled=false` overriding `ssl.bundle`
• `ssl.enabled=false` + `ssl.bundle=...`: the bundle is resolved and configured but then cleared, so no exception is thrown and SSL remains off

An earlier iteration applied the kill switch before the `orElse` merge, but that would have let a base-settings bundle resurrect itself through the null-bundle merge, so the clearing is deliberately applied after the merge.

Closes gh-51472

N/A

Yes — 4 new tests in `HttpClientSettingsPropertyMapperTests`:

• `mapWhenSslEnabledWithNoBundleDoesNotSetSslBundle` — enabled with no bundle leaves bundle unset (JDK trust material)
• `mapWhenSslEnabledUsesBundle` — enabled + bundle resolves the bundle
• `mapWhenSslDisabledDoesNotSetSslBundle` — disabled + configured bundle does not throw and results in no bundle
• `mapWhenSslDisabledOverridesBaseSettingsSslBundle` — disabled clears a bundle inherited from base settings

All 13 tests in the class pass, `checkFormat` passes, and the module compiles cleanly.

No.

[philwebb]

Thanks for the pull-request, but as mentioned in the issue comments #51472 is currently labeled as pending-design-work which means we aren't ready to accept contributions.