Add sending domain update and company info endpoints - #70

Changes

Adds the sending-domain update endpoint and the nested company_info endpoints, which exist in the API (per email-sending.openapi.yml) but were missing from the SDK.

• SendingDomains.update — PATCH on the domain, with UpdateSendingDomainRequest covering open_tracking_enabled, click_tracking_enabled, tracking_opt_out_enabled, auto_unsubscribe_link_enabled and inbound_enabled. The inner data class uses Boolean with @Builder and @JsonInclude(NON_NULL), so only the fields a caller sets are serialized and an explicit false survives.
• CompanyInfo / CompanyInfoImpl — new resource on client.sendingApi().companyInfo() with getCompanyInfo / createCompanyInfo / updateCompanyInfo. The spec path is /api/domains/{domain_id}/company_info and the account is resolved from the API token, so it takes no accountId.
• Response shape — the company-info methods return the CompanyInfoResponse envelope and callers read .getData(), matching Webhooks and the rest of this SDK rather than unwrapping in the impl.
• DTOs — model/request/companyinfo/{Create,Update}CompanyInfoRequest and model/response/companyinfo/{CompanyInfo,CompanyInfoResponse}.
• SendingDomainsResponse — add tracking_opt_out_enabled. The API returns it and update writes it, so callers could not read back what they had just set. Both domain fixtures were updated to include it.
• Wiring — adding the companyInfo field to MailtrapEmailSendingApi changes its Lombok @RequiredArgsConstructor signature, so MailtrapClientFactory.createSendingApi is updated in the same change.
• Tests / examples — CompanyInfoImplTest plus an update case in SendingDomainsImplTest, four new fixtures, a new CompanyInfoExample, an update block in SendingDomainsExample, and README entries.

update on the domain itself stays on the account-scoped /api/accounts/{accountId}/sending_domains/{id} route with the sending_domain wrapper, consistent with create / get / delete on the same resource; the API accepts both that and the spec's account-less form.