[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 dialing custom URIs/schemes from a "target" string

Requirements:

• update the url building logic in RPC and in-process modes to support custom URIs
• changes in options to allow specifying a custom target URI including scheme
  • support env-var based configuration as well, as with existing options
  • consistent with [flagd] Support custom schemes (name resolvers) java-sdk-contrib#995
• no breaking changes
• associated unit tests (e2e not necessary)

cc @pradeepbbl

[pradeepbbl]

@toddbaert I'm happy to take this as well