[flagd] Support custom schemes (name resolvers)

[toddbaert]

As described here, we wish to support custom name revolvers for additional control and configuration according to user's infrastructure. In order to make use of this, we must support the creation of channels created with NettyChannelBuilder.forTarget, and expose an option in the flagd that use a "target string" instead of a host/port combination.

Requirements:

• optional usage of channel built using .forTarget (see: https://github.com/open-feature/java-sdk-contrib/blob/4197d3231d68fb686b61ac1ca5eb8c278044a0e3/providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/resolver/common/ChannelBuilder.java#L30)
• changes in options to allow specifying a target string
  • support env-var based configuration as well, as with existing options
  • consistent with https://github.com/open-feature/go-sdk-contrib/issues/585
• no breaking changes
• associated unit tests (e2e not necessary)

cc @pradeepbbl