OpenThread C API Reference Updates

See "api: platform" and "api: user" labeled Pull Requests on GitHub for additional details on API Reference updates.

Backbone Router API added.
Network Simulator Platform API added.
otNetworkDiag* structs added to the General API for improved Network Diagnostic usability.
otThreadGetDomainName and otThreadSetDomainName functions added to the General API.
otCommissionerGeneratePskc function removed from the Commissioner API. Use the otDatasetGeneratePskc function in the General API to generate a PSKc going forward.
Router API updated for access to the address cache table.

Added OT_COAP_MAX_RETRANSMIT and OT_COAP_MIN_ACK_TIMEOUT macros to the CoAP API.

Updated the CoAP API:
- otCoapOptionIterator and otCoapTxParameters structs added.
- Added helper functions to encode CoAP block options.
- Added functions to specify per-message transmission parameters for CoAP requests and responses.

Server and Service configuration functions and structs moved from the Server API to the General API to support service registration by Leaders.
*RouterRoleEnabled functions renamed to *RouterEligible in the Router API to support REED enhancements.
otPlatLogv function removed from the Logging Platform API.
OT_TOOL_ALIGN macro removed from the Toolchain Platform API and from various struct definitions.

otThreadResetIp6Counters function added to the General API.
New functions for reseting counters and getting and setting maximum frame retries added to the Link API.

Split the Random API page into Crypto and Non-Crypto to align with source code.
otJoinerInfo struct and otCommissionerGetNextJoinerInfo function added to the Commissioner API.
otRadioCoexMetrics struct and otPlatRadioIsCoexEnabled function added to the Radio Types API.
Changed the return type of numerous APIs that always succeed from otError to void.

otPlatSettingsDeinit function added to the Settings Platform API.
otPlatUartFlush function added to the UART Platform API.
mTimestamp moved from the otRadioIeInfo struct to the otRadioFrame struct.
otNetifInterfaceId enumeration removed from the IPv6 API.

Added callbacks for notifying of state changes and joiner activity to the Commissioner API.
Compiler-specific function modifiers of OTAPI, OTCALL, OT_CALL, and OT_CDECL removed in conjunction with removal of Windows support.

otNeighborTableEntryInfo struct added to the Router/Leader API.
otDatasetCreateNewNetwork and otThreadGetRloc functions added to the General API.
otUdpSendDatagram function added to the UDP API.
Random API moved from a platform abstraction to a module, with improved cryptographic random number generation.
Entropy and Entropy Platform APIs added.

otPlatRadioGetPreferredChannelMask and otPlatRadioGetSupportedChannelMask functions added to the Radio Operation Platform API.
Removed physical layer channel functions from the Link API.

Added support for OQPSK modulation in the 915MHz band to the Radio Types Platform API and the Link API.
Various CoAP Secure API refinements.
New SLAAC module functions added to the IPv6 API.
otSemanticallyOpaqueIidGeneratorData struct and related functions removed from the IPv6 API.

otIp6SlaacIidCreate and otIp6SlaacUpdate functions removed from the IPv6 API.

UDP Receiver functions added to the UDP API.
otCoapHeader struct removed from the CoAP API. CoAP header information has been refined into the CoAP message.
The DHCPv6 Server API has been removed.