Stabilize useTransportSecurity API on ManagedChanneBuilder

[ramaraochavali]

This allows the consumer of a builder to swap back from plaintext. ManagedChannelBuilder.useTransportSecurity() is similar to ServerBuilder's method, but without args) method which will reverse the usePlaintext().

For Netty, useTransportSecurity() would be the equivalent of negotiationType(NegotiationType.TLS).

[ejona86]

API review:

• While we will be stabilizing usePlaintext(), we're thinking of leaving this one experimental for longer as there is not a strong need for the method since useTransportSecurity() is the default.
• We will be introducing a "replacement" usePlaintext()/useTransportSecurity() in the form of ChannelCredentials and making a decision for useTransportSecurity could easily wait until the new API is introduced
• The API docs could probably be improved a bit to make it clear that useTransportSecurity() is the default

[ejona86]

API review:

• ChannelCredentials is soon becoming stable, so we revisited this. We saw ~20 usages of channel.useTransportSecurity() internally, but none of them actually do anything as they are called on a freshly-created channel builder. The new ChannelCredentials API would seem to serve these users well in a "the code clearly shows TLS will be used" way.
• The ChannelCredentials API does not allow changing the credential after creating the builder, which is different from the current API with useTransportSecurity()
• A bit weird to consider stabilizing an API that most callers don't even need to call. But there's no rush to remove this API either. It could be perpetually-deprecated and not be an issue
• Server-side does not have an equivalent usePlaintext()

[larry-safran]

API review:

• useTransportSecurity is the wrong approach with the new API, should just pass in credentials when you create the builder.
• [Sergii] Have a fromBuilder that creates a new builder from another with overrides
  • Would be heavy and bug prone, so Eric recommends against
• Transport is selected based upon builder, so can't swap to different transport
• ManagedChannelBuilder new API throws exception if call usePlainText or useTransportSecurity
• Could put in text on old API to utilize useChannelCreds is preferred
• Stabilizing this creates consistency. Deprecated decision can be made later for whole old API.
• Everyone is fine with stabilizing it.