Send IS for adding MSISDNs only when required by HS

[jryans]

Cloned from element-hq/element-web#10599

Similar to email handing, MSC2263 records that m.require_identity_server will return a boolean that applies to both email and MSISDN cases.

Earlier in the project, it was unclear what would happen for MSISDN cases. As of this task, we we will only supply an IS to the HS in the following paths:

• Registration with MSISDN, but do not send if m.require_identity_server exists and is false
• Adding MSISDN to HS account, but do not send if m.require_identity_server exists and is false
• Binding MSISDN for discovery (request proxied via HS, always send IS)

This should match the behaviour with email.

[BillCarsonFr]

Done in sdk 66be4128b8aa4b768f009ed17c06ff5db3b76c61