fix: #355 Explicitly passing --project argument when starting emulator

[jainsahab]

fix #355

Context:
LocalDatastoreHelper starts a subprocess gcloud beta emulators datastore start to start the emulator, It is always starting the subprocess under the assumption that user have already set the project id property using gcloud config set project some-project-id. If the project-id property is set, it'll result in successful execution of sub-process. Otherwise it'll throw the following error.

➜  java-datastore git:(main) gcloud beta emulators datastore start
WARNING: Reusing existing data in [/Users/jainsahab/.config/gcloud/emulators/datastore].
Executing: /Users/jainsahab/Work/bin/google-cloud-sdk/platform/cloud-datastore-emulator/cloud_datastore_emulator start --host=::1 --port=8045 --store_on_disk=True --allow_remote_shutdown --consistency=0.9 /Users/jainsahab/.config/gcloud/emulators/datastore
ERROR: (gcloud.beta.emulators.datastore.start) The required property [project] is not currently set.
It can be set on a per-command basis by re-running your command with the [--project] flag.

You may set it for your current workspace by running:

  $ gcloud config set project VALUE

or it can be set temporarily by the environment variable [CLOUDSDK_CORE_PROJECT]

Note: To unset the project-id property, run gcloud config unset project

BlockingProcessStreamReader.java file from google-cloud-core library which is a java Thread ignores this helpful text and the subprocess's exit code to silently completes its own execution. Hence, no error was given to the user.

Solution:
As part of this PR, google-cloud-core library (covered in this PR) is out of scope, but we are explicitly passing --project argument now while starting the emulator so that it no longer expect the project-id property to set.
In addition to that, we are also giving the user the capability to set a custom project Id.

[jainsahab]

PR for the issue in google-cloud-core library is up for review too @ googleapis/java-core#1039.