Move ProtocolType and helpers to rtc_base/net_helper

The `ProtocolType` enum and its string conversion helpers
(`ProtoToString`, `StringToProto`) are needed by other build targets,
such as `webrtc_sdp`. Keeping these definitions in `p2p/base/`
contributed to a dependency cycle, which was further complicated by
existing dependencies involving `webrtc_sdp` and the types declared in
`jsep.h`.

`net_helper.h` (although ambiguously named - especially considering that
there's also a `net_helpers.h`) is where related protocol constants and
method are declared, that have the same dependency requirements as
ProtocolType. A potential future improvement could be to rename
net_helper.* to protocol_type.*.

Bug: webrtc:12330
Change-Id: I2b5e82d7ae613cb276ca42e48b2ee00ef48dcc32
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/411180
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45784}